Weak FM radio reception in 2010 Prius?

Pretty bad when you're away from a very strong signal, but noticeable even when the signal strength should be pretty good.

In my case the noise is primarily a frequent, intermittent "chuffing" sound as the signal fades, or gets interfered with, and returns, plus a lot of background hiss as the signal strength fades. The "chuffing" sound stops iof you stop the car; it's clearly motion dependent. As far as I can tell the radio doesn't attempt to switch from stereo to mono to regain the signal-to-noise ratio when the signal strength is weak, at least the "ST" indicator remains on, so this leads me to believe that it may be an interference problem, rather than something to do with signal strength..

My radio is the JBL system, not the bottom of the range system and not the top of the range system with nav, but I still expect to be able to listen to FM radio most of the time without background noise and signal interruptions. Every other car I've owned in the past few years has had an acceptable radio, the Prius is far and away the worst I've owned in maybe 30 years.

Jeremy

 

Extremely Bad. Worst kinda ever encountered. Switched on my map lights and my DAB radio just loses signal all together. So when opened my door or any door the DAB radio will lose signal and nothing, normal radio will have terrible loud pzzzsst sound for as long as the light is on.

 

We are all in the same boat.

I am feeling sorry for myself that I can't listen to the radio on this car and Toyota thinks thats acceptable.

 

I think the wrost thing is I've got the SatNav and the Rear parking camera Plus the self parking option. Otherwise I would have just change it to an after market radio.

 

I'm fascinated by the permutations of "poor radio quality". Mine is a pretty strong signal compared to my last car (I can hear it in areas it faded out before), but it is so full of static I can't listen to it anywhere.

(I took it in to the dealer, and after checking it out, he actually took me around the block in another 2010 and showed me "it was not just my car--that's the way the radio sounds in the 2010". I guess it was supposed to make me feel better that ALL of the 2010s sound the same!?!?!)

My question is: in all these very esoteric audio descriptions in this thread and others, is there anything that actually makes the static go away?

 

A quibble -- it isn't "static in the old fashioned sense, and it isn't related to receiver sensitivity. It seems to be related to signal processing.

It's a fundamental, duplicable, design flaw that is present in every vehicle with stock radios, and the only way it's going to get fixed is a class-action suit. With publicity, or the threat of same.

 

Sign me up NOW!!!

 

I'm inclined to agree, Toyota are unlikely to do anything unless pushed, hard. On the publicity front, it's worth remembering that one theory for the Toyota uncommanded acceleration fault on other cars was EMI. It's possible that EMI is the cause of the radio problems, so raising this again in the media wouldn't be something that Toyota would want to see.

I've complained to my dealer and asked them to take it up with Toyota, plus, by chance I received a "customer satisfaction questionnaire" from Toyota on Saturday, so I've made my views about the radio very clear to them on that.

I also agree that the problem isn't a weak signal, as such, as the radio makes odd noises on FM pretty much all the time, even when the signal strength should be OK.

I'm seriously thinking about making a head unit switch, rather than wait for Toyota to get around to fixing the problem, which, to be honest, I don't think they will, at least in any reasonable sort of timescale. The question is, will fitting a new head unit fix the problem, or is it an interference issue that's intrinsic to the car?

Jeremy

 

Picked up my 2010 last week and the issue with the FM reception is painfully obvious. I seriously stopped listening to FM and have been streaming Pandora. I have a Prius II. So, I ordered this:

http://www.amazon.com/gp/product/B0011UK2HE/ref=oss_product

 

I did a little thinking about this and a little Googling.

First the thinking:

1. There is no problem with playing CDs. Ergo the problem is not in the audio amplifier.

2. then I forced myself to listen to AM radio. It is a vast wasteland, but what audio quality problems I detected all seem to arise from studio equipment.

Then the googling:

The front ends of contemporary radio receivers are implemented as single chips. Here's a data sheet for one from NXP (Pilips):

http://www.nxp.com/documents/data_sheet/TEF6730A.pdf

The functional description is on page 1. There's a block diagram on page 5.

Depending on how nerdy you are, you can get tied up in the data sheet for days. For our purposes, the important thing is that there are only two output channels: IFOUT1 and IFOUT2, which I assume correspond to stereo left and right.

That means that everything downstream is the same, whether you're listening to AM or FM. (I also assume that the outputs are digital data streams and that they feed a DSP chip configured as an audio codec, meaning you can't just tap in there with headphones and listen.) If everything downstream is the same, regardless of whether it's AM or FM, and if AM sounds okay and FM doesn't, this chip must be.where the difference arises.

Except its a monolithic piece of silicon. Except for making it not work at all, it's hard to screw it up, and it's easy to see how well it works on the bench.

And there's plenty of competition on these things. This chip is simply the first one I found a data sheet for. Whether Toyota uses this one from NXP or another from StMicro, or one from a Nipponese company, they undubtedly tested them all on the vehicle platform and got acceptable performance. So how did they screw up? I don't know.

(If somebody has an HU they can open up and can read part numbers, please tell me what the chip with the biggest fanout is. My assumptions about what generation of front ends they're using may be off.)

 

Nice thinking, but there's nothing digital in the RF or audio parts of the radio. The front end chip outputs at Intermediate Frequency (the "IF out" pins), where the tuned analogue FM signal (now down-converted to the IF) gets amplified further. Following the IF amplification, the FM analogue signal gets demodulated. It's then switched into the left and right channels by demultiplexing the analogue signal (FM stereo uses a very old multiplex technique). Following the demultiplexing the left and right audio signals get routed to amplifiers.

I think that the problem is almost certainly in the RF front end design. Either the late front end/early IF stage AGC/limiting is severely flawed, or the broadband RF first stages are severely compromised in terms of their signal handling and cross modulation handling ability. From the symptoms, I'm inclined to the view that the problem may be cross modulation, or inter modulation, related, perhaps with one of the cross mod signal sources being from the car. If the front end is non-linear (quite likely in a cost-conscious product like this), then the likelihood of there being poor cross mod or inter mod performance is increased.

Such a problem may not be an issue in a "normal" car, as the 88 - 108MHz band is pretty quiet and unlikely to have any really powerful sources in it, other than genuine FM transmissions. The Prius may throw a spanner in the works, by stressing the front end of a normally good radio to the point where modulation products rear their ugly head.

Just a theory, but it seems to fit the symptoms.

Jeremy

 

Thanks for the correction, Jeremy. Let me check with you on perceptions. I think I'm hearing two different artifacts. One is a hissing on sibilants and other sounds with high-frequency components. The other is more random and sounds like, but probably isn't adjacent-channel leak-through. Are those good descriptions?

 

Pretty much so. I get a fair bit of high-ish frequency white noise (hissing), particularly when stationary, coupled with fairly random noise bursts that make a sort of "chuffing" sound. The latter sounds like multipath reception, the sort of phase-cancellation noise that you get on a poorly designed radio when aircraft fly overhead.

What I find odd is that the noises are present to some degree even when I know (based on the reception in other cars) that the signal in that area is OK. In fact, the noises when the signal gets weak, in bad reception areas, are subtly different to the noises when the signal should be OK.

Jeremy

 

Sorry about that, but I have to agree with Jeremy. Hey, that's my kind of data sheet!

However, the IF1 and IF2 output are Intermediate Frequency outputs, probably to be fed into a selective (band limiting) filter at 10.7 MHz (a very common IF frequency). The band limiter passes a (relatively) narrow bunch of frequencies: For AM it'll be roughly 30 kHz wide or so, to handle the carrier and 15 kHz upper and lower sidebands; for FM it'll be 200 kHz wide, which is the deviation of an FM signal. do not know for digital radio.

After the bandpass filter (which is sometimes quartz, sometimes some kind of audio resonator, etc.) the signal would then go back into the device. At this point you'd get into some Automatic Gain Control that would attempt to level the amplitude of the IF signal to some level; you'd then go back out of the device and into a demodulator. The demodulator would convert the thing to audio or whatever.

Actually, reading over the data sheet a bit they state that the output of this thing could then go into an ADC (analog to digital converter) for further processing. That's the fun part: The ADC would be sampling along at $DIETY's own speed, then doing digital signal processing math functions that would then demodulate the IF signal. Which, I imagine, is probably cleaner than trying to do it the old-fashioned way with funny-tuned LC circuits (for FM) or diodes and filter caps (AM), and who-knows-what for satellite.

It does look like a fancy mixed-signal IC for most of the front end. But there'd be a ton of supporting components around it. Fun.

KBeck

 

Jeremy,

Let's try it this way. I'm going to talk about noise floor.

All radio systems have a noise floor. Sometimes its caused by the environment; sometimes its caused by the electronics. Thermal noise and noise from natural sources dominates below 50 Mhz or so. If you're on the AM radio band and you tune to a place where no radio station is, you'll hear it: That wonderful hiss.

A spectrum analyzer displays frequency across the X axis and signal strength of whatever across the Y axis, and sweeps from left to right. (An oscilloscope, by comparison, has time across the X axis and signal on the Y, but, usually, also sweeps left to right).

If you look at the AM radio band during a good, solid blackout, you'll see a more-or-less straight line. Let's say, just as an example, that that line, for a 30 kHz bandwidth in the AM band, is at -30 dBm. (That's a power level 1000 times less than 0 dBm, which is one milliwatt.). So, the level of the noise signal in this example would be at one microwatt.

Next, suppose that we turn on some AM station 40 miles away. On Ye Spectrum Analyzer you'd see the AM station come up, centered on its transmitting frequency, and about 30 kHz wide. (If the guy stops talking, the width goes to near zero, but that's the audio sidebands..)

Let's suppose the peak of the signal is at -20 dBm, or 100 times less than a milliwatt, at 10 microwatts.

We'd now say that the signal-to-noise ratio (SNR) is the difference between the noise floor and the AM signal. In this case, about 10 dB.

As it happens 10 dB is a plenty good SNR for AM radio. You might hear a little hiss.

However, suppose there's a solar flare. The AM band gets maxed out with noise, which is now lots bigger than your AM station, and bye-bye music. In fact, when the earth is facing the sun, the solar irradiation does cause a lot of background noise. (E and D levels in the ionosphere, but I digress..). When the sun goes down the noise levels go down, too, and suddenly more distant stations can be heard. You get skip, too, but that's another issue.

Now, up at the FM band, there's still that environmental noise. But, that noise goes down as one goes up in frequency. Unless you're running around with liquid nitrogen cooled germanium transistor front ends, your radio simply won't pick up that noise - because the thermal noise of electrons moving around in the resistors, capacitors, and transistors will be 'way larger than the environmental noise. (With the liquid nitrogen stuff, and liquid helium for the real geeks, the thermal noise gets reduced far enough that a first stage amplifier can bring up the environmental signals. Good for people in astronomy and the like, but rather impractical and unnecessary for people with car radios.)

Now, relative stuff. In the AM area, the noise is huge and dominates. Hence, AM transmitters put out mongo signal levels in order to be bigger than the noise. (Those that stay on at night, when environmental noise levels are less, tend to reduce power levels; there's lots of reasons why, but saving on the power company bill is one of them.)

FM is different, though. The environmental noise is not really there. How far you get is partly a function of how big your power level is, but, really, with FM you're more-or-less limited to line of sight, plus maybe some ground wave. Hence, I would guess that in general FM reception is more likely in the 30 dB SNR or less range for a lot of radio stations, and I wouldn't be surprised that in any one given spot 10 dB SNR radio stations would not be unusual.

Now, the demodulation of FM is a little funny. With AM, as you decrease the SNR, you can perceive the AM noise around 10 dB SNR; as the AM station gets to lower SNRs, you can still hear the signal. Pretty much the noise comes up at the same rate the SNR goes down; at 0 dB SNR (noise and the AM signal are equal) the human ear can still pick out words and hear a tune, but that's because we got a really good DSP built into our heads.

FM is different, though. You'll have to take my word for it, but at a given radio SNR, FM has a better audio SNR than AM radio has. However, when the radio SNR gets below 3 dB or so, the audio SNR snaps down to rotten levels much quicker than the audio gets worse on a decreasing level AM signal. And, yes, you can hear it when it happens. (If you really want to go long distances, like hams do, they tend to use single sideband, a variant of AM. Much better in lousy SNR situations.)

Now, what you guys are kvetching about sounds a lot like picket-fencing, the kind of noise you get because of the reflected and direct radio signal interfering with each other. Out in the boonies that's expected as you get farther from the radio station. In town, not so much. So everything points at a reduced SNR of the radio signal.

So, what does that?

1. The amplifier in the antenna is screwed. Either it's flat-out busted, in which case it's a great attenuator and giving the radio nothing to work with, or it's mis-built. Turns out that the noise performance of the first stage amplifier in any radio is critical for the noise performance of said radio. Mis-bias that first transistor and, even if it gives you gain, it can give you too much noise, too.
2. I'd hate to say this, but that first amp can be badly designed. That first amp's most important job is to get about a factor of two or three gain (that's not much) with as low a noise as possible. The idea here is that, even with a strong signal as you drive down intermod alley between all the FM transmitting antennas, you're not going to overload the output of the amplifier. However, if somebody designed it with a gain of 10 or 20, that would be Bad News. Driving down intermod alley would cause the output of the amplifier (which is wide-band) to be rail-to-rail on the output in response to some FM station you're not listening to, thereby screwing the signal you are listening to. We want small, linear, low-noise amplification.
3. Either of the above could also be true in the radio front end, as well.

None of this stuff is radio rocket science. Somebody trained up in radio repair, like we used to have in ye olden days when we didn't just throw the radios out, could figure this kind of junk out 15 seconds after getting the test equipment warmed up.

It's not like people have gotten dumber or anything. Heck, I betcha there's auto mechanics in the dealerships who can break down an automatic transmission to parade rest, fix the busted pieces, then put it back together again. It's just that finding people who can do that, commercially, for radios is pretty difficult. The radios, even the expensive ones, are cheaper to toss than repair. I don't know this for sure, but I bet that all those little commercial car radio "repair" places would be lucky to have a voltmeter in the shop, it'd be amazing if they have a 'scope, and I think Marconi would roll in his grave if they had a spectrum analyzer. But that last is what would be needed in order to figure all of this out.

I think I'm going to build that little test box. Schematics posted in a couple of days.

KBeck.

 

Just to muddy the waters, I've no doubt but that Jeremy is right about the functioning of the NXP chip, but maybe if I explain why I thought the IF out was digital, it will trigger some ideas.

What threw me for a loop was upconverting the AM IF to 10.7 and feeding it out the same port as the FM IF. You need a discriminator for FM, while any old nonlinear device will do for AM -- but why output the AM IF at 10.7? To save two pins? Because it makes the post-detector LP filter smaller and sharper-tuning? Maybe. It just strikes me as curious.

OTOH, with digital output, followed by a DSP codec implemented in a TMS320, it would have been simpler, and to me, the NXP data sheet was ambiguous about the output mode. I got an impression it was high speed CANBus.

Alas, digital would have opened up some possibilities -- a poorly designed anti-aliasing filter, a noisy amp in a Chinese knockoff of the NXP part reducing th ENOB of the data converters -- the trouble is, I know what aliasing is in theory, I don't know what it sounds like in practice. maybe it doesn't sound anything like what we're hearing.

It would still be interesting to know the actual part number of the Toyota front-end chip, however it works.

 

kbeck,

I used to design RF front ends many, many years ago, albeit at UHF and microwave frequencies, so understand the core noise issues but I'm not convinced, based on the behaviour of the radio, that this is solely a noise floor issue.

Just listening to the radio can determine some useful things. For example, despite the much higher RF background noise in the AM band, plus the inherent sensitivity of AM to noise, particularly impulsive noise, the radio doesn't seem too bad on AM. Part of this may be due to the tendency for medium frequencies to follow the Earth's curvature slightly, but it must also indicate that the broadband noise from the car itself, at least at medium frequencies, is acceptably low.

Now, moving up to the VHF FM band, we have a much better RF environment, plus a modulation method that is inherently better at rejecting impulsive noise and which tends to be less sensitive to noise generally. In fact, if the radio is properly designed, then it should be hard limiting the IF to get a pure FM signal, with no AM components, out to the demodulator. That way the radio will be insensitive to non-frequency modulated interference. However, some FM front ends have a rather nasty quirk, which makes them rather sensitive to relatively low level signals that are within the front end pass band. Because such signals aren't commonly found in ordinary cars, this isn't a problem. It may be for the Prius though, as it is far from "normal" when it comes to it's electrical systems.

The issue here is that the radio seems to be picking up false FM components and demodulating them as noise in the audio path. We know this can happen from multipath reception, where the same signal arrives at the front end from a direct and a reflected path. If the reflector or the receiver is moving, the relative Doppler on the two signals can create a false FM signal, the common "aeroplane effect" noise heard on poor FM receivers. However, a decent front end should frequency lock on to the strongest signal, so rejecting the false, reflected signal. It seems that the Prius radio front end isn't much good at this, based on what we're hearing.

One possible reason for an FM front end not locking to the desired signal is when they get swamped by a non-FM signal. One consequence of this is that radio can produce odd artifacts as a consequence of inter-modulation between the Doppler shifted recieved FM signal and the fixed frequency interfering signal.

A good example of this from many years ago was a BMW I owned, one of the first BMWs with digital fuel injection. My local FM station broadcast on 96.0 MHz. The BMW microprocessor ran at a clock speed of nominally 8MHz. The result was severe radio interference at 96MHz (12 x 8MHz), but a radio that worked OK on other frequencies. BMW eventually recognised the problem (primarily because there were other EMI problems with the fuel injection system) and fixed it. I actually measured the 96MHz signal from the car with a spectrum analyser and it was at a very low amplitude, lower at the car aerial than the strong signal from the local FM transmitter. It was still strong enough to generate problems for the (admittedly poor) FM radio front end.

Jeremy

 

Hokay. Let's see if I can straighten you out on this one.

Basic radio design. The front end of a radio, up to and including the mixer, is typically wide band. The output of the front end doesn't have just one radio station in there. Depending upon the design, it's got anywhere from dozens to hundreds.

What you want is just one radio station. So, you pass this wide-band signal through a narrow, band pass filter that selects just one radio station. Said filters:

1. Aren't cheap, and are a major cost component of the radio.
2. They're not just a tuned L-C circuit. To get narrow bandwidth, the math of the tuned circuit means you need high "Q", or quality factor. As a first order approximation, Q = Xl/R; you can (typically) get a Q of 10 or maybe, on a good day, 100 with nice, standard components. But what you need is a Q of 1000+. For that you use a specialized quartz bandpass filter. (Unless you're Collins Radio, using an IF frequency of 455 kHz, and you've got a strange mechanical resonator. Fun.)
3. Quartz crystals for radio use use the piezoelectric effect, where mechanical compression causes voltage variation and vice versa. These things actually vibrate at the frequency in question; 1 MHz quartz crystals are relatively thick, and the limiting factor on how high a frequency you can get out of a piece of quartz is how thin you can make it before it snaps in two. Typical quartz crystals top out around 50 MHz or so in fundamental mode, although you can have real fun with overtones (think about swinging a rope tied to a tree with multiple nodes).
4. Between one thing and another, quartz oscillators aren't really tuneable that much. By putting a capacitor in parallel or series with one you can tweak the quartz one way or another a bit. So, what this all means is that, very typically, your IF frequency is fixed.
5. 10.7 MHz was initially picked, I think, in large part because it was smack dab in the middle of the useable quartz filter range. There's other reasons too: Not too close to other commonly used frequencies; in the middle of the range where resistors are resistors, capacitors are caps, inductors act like inductors (think I'm kidding? Ask about stripline design some day..), and the components involved are all relatively small (the higher in frequency you get the smaller the parts tend to get). Once people began using this area in large numbers, a bandwagon feedback loop started up resulting in gazillions of 10.7 MHz quartz filters, cheap, and of decent quality. You can get a quartz filter for any frequency you like. But if you're the only user, it'll cost you 100x (at least) the cost of a standard filter.
6. So, if you already got one filter chugging away at 10.7 Mhz with a 200 kHz bandwidth, it makes all sorts of sense to put in a second with with a 30 kHz bandwidth for the AM and switch between them, depending upon which band you're playing with.
7. Finally, the radio is picking up signals whose least amplitude is measured in the microvolts and, by the time you get to the final detector, is measured in volts. That's a gain of a million or so. In order to prevent overloading, you don't use a heck of a lot of gain before that quartz selective filter; after that filter, that's where the majority of the gain exists - and you're only amplifying the signal you want, not everything else in creation.

This IC has the transistor guts of the radio front end. The quartz (or whatever) resonator; front end band pass filters (L's and C's, I note), feedback for the tuning PLL, and lots of other good stuff are all outboard. It has cool features, like an I2C bus for tuning and all that, highly integrated, and probably costs $5-$10 in quantity. It's certainly possible to build a discrete component radio with a bit better specs - but you'd probably spend ten times the $5 in component costs, you'd have a lot more parts to solder down (big reason why IC designs are more reliable, fewer solder connections!), and probably wouldn't get that much an improvement in specs.

Now, let's talk about Evil. IC's are susceptible to ESD when being built; it's possible the radio manufacturer is not testing every radio going out the door in order to save costs, figuring that people would notice first; and there might be a batch of bad ICs. Stuff happens. I personally favor the bum antenna amp possibility myself since that sounds like a "build-it-and-don't-test-it" item if ever I heard one, but I'm willing to listen to argument.

KBeck.

 

FWIW, here's the data sheet for the companion back end to the NXP front end I posted earlier:

http://www.nxp.com/documents/data_sheet/SAA7706H.pdf

I'm not saying the Prius radio uses these, but if not, the architecture is likely to be similar. This is 2006 technology, BTW.

In this chip, the tuner IF is digitized immediately, and an ARM core handles the DSP from there.

So my suspicions that the problem could be in the digital domain aren't totally nuts.

 

Jeremy, sorry I missed your post. Here's a direct reply!

Jeremy, I think my argument here is that the overall signal level in the AM band is just, well, huge. The environmental noise is big; the AM radio stations, in order to work, are even bigger. So, if (for example) we've got a bum antenna amp, 20 dB of attenuation will bring things down a bit, but we'll still be quite a bit above the noise floor of the radio for both the external noise and the signal. What with AGC and all that you might not even notice the reduced level AM signals. Even at night, when the ionosphere quiets down, the ambient RF noise is still well above the receiver noise floor.

This is where I think I disagree. The argument is that for all intents and purposes the environmental noise has disappeared. So, the noise floor is, basically, the noise floor of the receiver. Now, if I was the FCC or the guy responsible for paying the electric bills at the radio company, I'd make sure that my signal had a decent SNR around the limits of my coverage area, but no more than that. Hence, I'd argue that overall RF signal levels would be less. I could be wrong on that; but I think I've heard of 1 MW AM radio stations; I don't think that FM gets up there quite as much.

Hence, a 20 dB attenuation in the guessed-at bad antenna amp would definitely bring a signal with a 20 or 30 dB SNR (which would be normally beautiful for FM reception) down to 0 to 10. Now, as you probably know, the ratio of RF SNR to audio SNR in FM is usually wonderful, and is the primary reason why people FM has the (deserved) rep for high quality audio. I agree with you about amping the FM signal until it limits and all that. However, when the SNR on an FM signal gets down into the single digits, the audio SNR gets bad, very fast. I got graphs showing this in my communications textbook, not to mention some serious lab time fooling around with this as an undergrad! If my memory's not shot, an AM signal with 0 dB SNR at the radio side has, pretty much, 0 dB SNR at the audio side, and they track together right down into the weeds. On the other hand FM at something like 10 dB SNR at the radio side has like a 20 dB at the audio side - but at 0 dB SNR radio, the audio goes to -20 dB SNR!

What this means is that when you get a dropout on the verge with FM it really drops out and is very noticeable. That's why hams with narrow band FM in the 144 MHz band (2 m) know all about picket-fencing, and you get the same effect with FM at 100 MHz.

Since people say the signal gets better when they move the car ten feet, well, to me that sounds like low signal amplitude against the car radio's noise floor. (There not being much else up there that makes noise!)

I do not know. Enough delta-V to make a separate frequency? Wowzers. Lo these many years ago I worked for a company for a summer one year in upstate NY on proximity fuzes for artillery shells. I vaguely remember 20 kHz doppler on the homodyne radar, but we're talking here about a shell blizting along at $DIETY's own speed. But FM runs 200 kHz deviation.. Haven't thought about this issue that much.

The following year I had full time employment at Harris RF Communications in Rochester, working on, believe it or not, hardware for the original AMPS cellular telephone service. It was digital work - in fact, I walked in there in a swoon, thinking I was going to do pure analog, and the first question I got asked was, "Do you know what a Karnaugh map is?". Ah, well: 8 months later I had me a CMOS gate array that could decode the 10 kb/s AMPS and hand said data stream off to a processor. But this is when I found out about picket fencing. A decent model for the RF environment in a city has the amplitude of the received signal following a Rayleigh distribution. What that means is that most of the time the signal looks nice and healthy. But every so often, more so as the signal amplitude comes down, you'd get a pocket of no signal, narrow in terms of time, that would drop down 10 or 20 dB, easy. And, yeah, it's fun building a PLL that can hang in there and stay on the right phase with ten to twenty missing clocks!

I agree with you, in general, about intermod. Doppler, I don't know. But, if you think about it, all radio receivers, AM and FM, have a spec for 3rd-order intercept, and that's intermod inside the radio because of increasing nonlinarities as the internal signal levels get bigger.

I've said it a couple of times, so I'll ping it again: a badly biased amplifier (I'm thinking of you, Mr. Antenna Amp) could generate noise, intermod, etc., and Do Evil. So I think I agree with you here.. But maybe not about the Doppler.

Ha. Love that last one. I remember when I worked at Harris just how crazy the RF engineers got trying to set the various oscillators at different places so the beat frequencies between the oscillators, the beat frequencies between the beat frequencies and the oscillators, and the beat frequencies between the beat frequencies and the beat frequencies, all of them, wouldn't somehow work their way into the desired RF band of interest or into the IF. (Technical name: Spurs. The name those engineers had for them? Unprintable.)

I suspect that BMW might have had more than disgruntled customers to contend with, though. The FCC takes a very dim view of unwanted RF radiators in the commercial radio/TV bands. Heck, so does the EU and, before them, the Bundestadt. I guess it might not have occurred to BMW that they needed to stick a car into an EMI chamber and check for emissions, seeing as your standard spark plug/carburetor/mechanical fuel injection kind of car doesn't tend to emit in the RF. Fun.

KBeck.