[HN Gopher] How to reverse engineer an analog chip: the TDA7000 ...
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How to reverse engineer an analog chip: the TDA7000 FM radio
receiver
Author : nynyny7
Score : 35 points
Date : 2025-08-02 15:59 UTC (7 hours ago)
(HTM) web link (www.righto.com)
(TXT) w3m dump (www.righto.com)
| kens wrote:
| Author here for if you have questions on this chip...
| magnat wrote:
| The separate noise source is a bit of surprise here. Why is it
| necessary? Wouldn't RF noise produce same results?
| wkat4242 wrote:
| It depends, if the RF frequency you use has a signal on it
| then it won't be random so it's not really noise. I wonder
| why they need a noise generator in a receiver chip though..
| They're usually used for crypto stuff.
| CamperBob2 wrote:
| It's to provide "comfort noise" when the correlator
| indicates a missing or mistuned signal.
|
| Muting the audio would make more sense -- and would
| certainly have been familiar to the CB[1] radio operators
| of the day in the form of a squelch effect -- but this chip
| was targeted at consumers who expected it to behave like a
| conventional FM radio.
|
| 1: An early incarnation of social media, for better and
| worse
| kens wrote:
| I'm not sure what the FM demodulator produces when it's
| mistuned, but I'm guessing that you'd get pretty much no
| output, rather than white noise (since there's no frequency
| for the demodulator to lock onto). The problem for the user
| is that you wouldn't know if your batteries are dead or if
| you just haven't found the station. By adding a "hiss"
| between stations, the radio has better usability
| magnat wrote:
| If RTL-SDR is a good reference - when demodulating FM it
| produces pretty much the same noise you'd expect from a
| mistuned oldschool radio.
| CamperBob2 wrote:
| In a conventional radio, yes, but I'll bet this approach
| would sound incredibly awful if mistuned.
| rep_lodsb wrote:
| I wondered about this too, but from the linked articles it
| seems to be designed that way in order to make it more user
| friendly: when not correctly tuned to a station, it outputs
| the artificial white noise instead of a possibly distorted
| signal from a nearby frequency (or just silence if the
| demodulator can't lock on to anything).
| contingencies wrote:
| Hey Ken, great read as always. I wonder if in future you would
| consider doing an overview of the various early radio chips and
| their evolution. I recall recently reading some HAM projects
| and understanding that a lot of the later radio chips were
| clones of earlier designs. Given your suggestion that this
| earlier period of integrated radio innovation is 'low hanging
| fruit' in terms of RE-friendliness, it should be an interesting
| read and I'm sure a very large number of radio enthusiasts
| would love to see your insights.
| CamperBob2 wrote:
| The correlator is interesting. I don't see how it works. In the
| perfectly-tuned case, how does delaying the signal by half an
| (IF?) period and inverting it yield a match for the original
| signal? Inversion isn't the same as a delay.
|
| I guess the idea is that the 70 kHz IF is effectively sampled at
| 2x the necessary Nyquist cutoff needed for 15 kHz baseband audio.
| So the signal content at half the period can be relied upon to
| match after an inversion and delay, assuming it was (a) band-
| limited at the source (or by the clever deviation-reduction
| scheme), which it would be; and (b) tuned correctly.
| kens wrote:
| The application note gives more details [1], but I find it a
| bit confusing. The idea is that as long as you are within about
| +/- 100 kHz of the station (a wide range), the radio will lock
| onto the right frequency (because of the frequency-locked
| loop), giving the nominally 70 kHz IF. Since the 70 kHz signal
| doesn't vary much over a half-wavelength (as you said), the
| correlator will be happy. The correlator will still stay locked
| as the IF varies +/- 15 kHz with the audio signal. (The
| correlator doesn't require a perfect match, just mostly
| matching.)
|
| The problem is that if you mis-tune the radio by 100 kHz or so,
| the FM detector will give you an output, but it will be
| distorted. The issue is that the FM detector is linear over a
| small range, but outside that range, you get non-linear side
| lobes. So if you tune to a side-lobe frequency, the radio will
| lock onto the frequency, but the output will have harmonic
| distortion. In this case, the IF frequency is way off from 70
| kHz, enough that the delayed signal and the inverted signal
| don't match at all, so the correlation fails and mutes the
| audio. Then you'd re-tune and find the right frequency.
|
| [1] See Figures 8-12. Link:
| https://www.tel.uva.es/personales/tri/radio_TDA7000.pdf
| gblargg wrote:
| > When the chip is not tuned to a station, the chip replaces the
| audio with a white noise source.
|
| Wow, I always thought the background noise was just natural when
| it was off-station, like on a TV.
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(page generated 2025-08-02 23:01 UTC)