The TDA7000 FM Receiver
 

My attention was first drawn to this IC when Elektor in December 1983 published a project called "Personal FM". Then in the late 1980's, Tandy were selling this IC for about $12. I didn't really take much notice of it until around that time when I was looking for simple VHF FM receiver designs, that would function better than super regen circuits. (This was before I discovered the Pulse Counting receiver design). I dug out the Elektor article and was intrigued at how this IC functioned and the lack of coils and alignment usually associated with superhet receivers. Looking at the Philips data with its mention of 1.5uV sensitivity also got my attention.


The December 1983 issue of Elektor was my introduction to the TDA7000.
 


Taken from the 1989 Tandy catalog.

Background
The TDA7000 was patented by Philips in 1977 but was not announced until January 1983. There's some interesting information regarding the history in this article.
What is unusual about this IC is how it operates. It is a proper FM superhet receiver, with the usual local oscillator, mixer, IF amplifier, limiter, and phase detector. The difference is that there's only one tuned circuit; the local oscillator. The TDA7000 relies on a low IF so that ordinary Op Amp circuitry can take care of the gain and bandpass characteristics. The use of a low IF in an FM receiver is not new and dates back to the valve pulse counting receiver design. The convenient aspect of this is that ordinary 'audio' circuitry used for the IF amplifier is non-critical, no alignment is required, and there are no inductances or ceramic filters. Pulse counting receivers have used an IF of around 200KHz which can accomodate the +/- 75KHz deviation of the FM signal comfortably.
However, the TDA7000 has an IF of only 70KHz. A fully modulated signal would therefore sound rather distorted. So, how can this IC work, with its 70KHz IF?
It's quite simple, in that there is what Philips call a Frequency Locked Loop. Basically, the local oscillator is shifted in response to detector output so that the bandwidth of the mixer output is never more than +/- 15KHz. It is actually compressing the frequency range of the modulated signal.


Original prototype TDA7000 receiver encapsulated in resin, powered from three NiCd cells. It has tuning indication (see application notes). This version does not use the RF input bandpass filter..

The muting or squelch feature is novel to say the least. Its main purpose is to prevent signals being heard if the receiver is mis-tuned. This is because reception can occur at more than one tuning point, and the user may not be aware that the receiver is not tuned to the correct one, with inferior reception being the result. Although as far as the user is concerned, it performs like any other muting circuit does, the TDA7000 provides an artificial noise generator, so that the receiver still sounds alive while tuned off station. In this regard, it tunes like a 'normal' FM receiver with the white noise present until a station is (correctly) tuned in.
If the artificial noise is not required, just remove the .022uF condenser at pin 3. Not all Philips data sheets show it, but connecting a 10K resistor from the supply to pin 1 will disable the squelch.


Block diagram of the TDA7000 as used for a typical FM receiver. Audio output is around 75mv.

At this point I recommend reading the Philips application notes, AN192.pdf, followed by the data given by Philips Components. They give a good background to the design and use of this IC.  For curiosity value, have a look at AN193.pdf for the application notes regarding Narrow Band FM. In case you were thinking a low IF would be suitable for a NBFM receiver, you're right. Apparently, the TDA7000 is not suitable for stereo, although I have seen one circuit where the output was fed into an LM1310 stereo decoder. It is not known how good the channel separation was.
It is interesting to speculate as to why Philips didn't use a pulse counting detector. After all, the low IF is ideal, and pulse counting technology had been around for a while.
As Philips (now NXP) considers this IC obsolete, any links to the application notes on their site no longer work, but these are provided below:


The TDA7000 starts a family!
Philips didn't stop with just the TDA7000 in its 18pin DIP package. Next came the TDA7010T which is the surface mount version. It comes in a 16pin SMD package. What of the other two pins? Well, the artificial noise generator has been dispensed with and so has the connection to one of the IF filter capacitors. The latter is a bit odd; I'm not sure if it could be dispensed with altogether or if they managed to fit it inside the chip. The data for both chips is the same apart from that.

Next comes the TDA7021T which is also surface mount but is stereo compatible. Lastly comes the surface mount  TDA7088T which is mono only but has a search type tuning and works on 3V. The TDA7000 is one of those IC's like the 555 that is a brilliant design with so many uses, but never really found its way into commercially made products. Instead, it seems that it's kits and other homemade gear that have kept it going. I have never seen the TDA7010T or TDA7021T used in anything commercially made, or even a kit. The TDA7088T however does exist in some of those miniature keyring FM receivers that are popular of late.
One could consider that the TDA7000 and derivatives are in effect the VHF FM equivalent of the ZN414; a 'minimal tuned circuit - no alignment' receiver IC for simple or miniature receivers.

R.I.P.  TDA7000
The TDA7000 is no longer being produced by Philips, having being withdrawn from manufacture, December 2003. Actually, it's a pretty long production run when you consider it was produced for 20 years, and it's taken nearly another 20 years for stocks to start drying up. So, if you want to play around with this IC, keep in mind that there won't be any new stock from Philips. The last TDA7000's I bought were made in 1994.  As DIP gives away to SMD packaging, it is unlikely it will ever be cloned by Asian manufacturers. There are stocks of the IC still available, but they are not cheap.
The SMD versions, TDA7010T, TDA7020T, and TDA7088T were kept in production for a bit longer, but these too are now out of production.
However, there are Asian produced clones of the TDA7088T in current production. These are identifiable by '088' in the number, but a different prefix; e.g., CD9088, SC1088, etc. While these are mostly used with push button auto search tuning, ordinary variable capacitor tuning can be used instead. The only catch therefore for the home constructor, is that they're SMD. These '088' IC's are prevalent in small inexpensive radios found in $2 shops, and appear in various eBay kits. Some are auto scan FM only, some use a variable capacitor, and where AM reception is included, a TA7642 (ZN414 clone) circuit is used.

There is another IC that appears to work on the same principles; Sanyo's LA1800. This IC can drive headphones directly, and also includes an AM receiver section which is a simple TRF circuit, like the ZN414 or MK484. It is not surface mount. However, it appears difficult to obtain, and no detailed data has yet been found.


Constructing a TDA7000 receiver
I bought my first TDA7000 IC in 1988 and tried to build up a receiver on a piece of matrix board. Here I learnt the first thing of importance. Layout and groundplanes are critical to using this IC. The Philips data gives a PCB layout and this should not be altered too much. Of course, my matrix board receiver didn't work properly.


Bought from Tandy in Chatswood; like all their components, packaging was excessive. An abbreviated reprint of the Philips AN192 application notes was included.

Soon after, Electronics Australia did an article (June 1988) with a TDA7000 and LM386 for the audio. So, I purchased the PCB and constructed just the TDA7000 part. I didn't think much of the LM386 (and still don't) so I made a two transistor class A amplifier on another PCB instead. The other alteration was to use a BB809 varicap diode for tuning. I didn't like the idea of EA using a trimmer capacitor.

Electronics Australia for June 1988 presented the first Australian constructional article using the TDA7000. Note that C6 is designated twice.


This is my adaptation of the EA circuit.

This receiver certainly worked and was a good introduction to the TDA7000.



Mains operated TDA7000 receiver
Around November 1990, I built a mains operated TDA7000 receiver. This used the EA June 1988 PC board, but fed a 6AV6 and 6DX8 amplifier in a plastic box. A 6X4 half wave rectified the mains with heaters powered off my original DSE2155 transformer. Of course being a live chassis set, precautions were taken. External screws were nylon, a mains rated transformer was used for the output transformer, and 400V isolating condensers were used to connect the aerial. Later, I removed the 6AV6 stage as the audio gain was much higher than needed. During April 2020, I rebuilt the receiver to use a 6AU6 and 12AU7 which eliminated problems caused by the use of a pentode output stage. A new dial was also made up.
RF performance is good. I can receive 2NUR from 135km away on a piece of wire. 2NUR is on 103.7MHz and is a weak station. However, there are two strong Sydney stations on 104.1MHz and 103.2MHz. The TDA7000 circuit can just separate these without interference. Article on this receiver here.


TDA7000 receiver works from 240V mains.


Circuit of the mains operated receiver.



The Silicon Chip November 1992 TDA7000 receiver

I built this receiver during 1995. The circuit is very typical using an LM386 for the audio stage. The LM386 is a very noisy IC and I'm certainly not fond of it.
Note that the artificial noise capacitor at pin 3 is not included in this circuit. When the "mute" switch is on, the receiver is silent between stations. If the capacitor is included, a slight rushing sound is heard when the squelch is activated.
Originally I built it in a plastic box with speaker, but wanting to make it smaller, I built a new enclosure out of aluminium, with a Lexan cover. (January 2004). I didn't bother including the speaker as I seldom used it. However, once I'd done this, the performance seemed very poor. Sensitivity was really bad. Eventually I noticed that it seemed like some sort of spurious oscillation was going on. Bridging the negative battery terminal straight to the case brought up a huge improvement, and a permanent cure was made by connecting the PCB groundplane to the chassis directly, not just relying on the headphone socket. It just goes to show how finicky things are at VHF.


I mounted 4xAA cells on the PCB where Silicon Chip intended the speaker to be placed. The squelch switch was not used; instead the squelch is permanently disabled. The telescopic aerial extends to 75cm which is a quarter wavelength.

Aside from the noisy LM386, performance is very good. In this construction, the squelch is permanently disabled. As such, there is no point including the noise generator capacitor.


TDA7088T Receiver.
I found this in a $2 box at the Jaycar stall at the Wyong amateur radio field day 2004. When I realised what it was I had to have it. So how did I know it contained a TDA7088T? First thing that gives away this IC is the "scan" and "reset" buttons. Through the tinted plastic case I could see it ran off 3V (another TDA7088T characteristic), and just the lack of peripheral components (ceramic filters/IF tansfromers) around the surface mount IC.


Those earphones are awful.

This tiny receiver is not much bigger than an AA cell. It is powered off two LR44 button cells, which are expensive and I assume wouldn't last terribly long. I'll be on the lookout for LR44's at the markets and $2 shops now that I've got this radio! As with all these sorts of radios, the headphone lead functions as the aerial. Supplied with this receiver were a pair of those awful "in-the-ear" type of miniature type earphones. Apart from the appalling sound quality, they are insensitive, unhygenic and dirty, fragile, and do not block out external sounds. So, I use the normal kind of headphones instead.
The enclosure is all clipped together, and once I'd opened it, sure enough,  a TDA7088T was visible.
The audio amp appears to be one transistor; ie. single ended class A. I don't know what current it's drawing so I can't say whether it's consuming much more battery current than a class B amp would. In any case I would prefer AAA cells rather than the LR44's.


Opened up, this shows two LR44 cells, the switch and headphone socket. Note the two RF chokes to allow the headphones lead to be used as the aerial. The TDA7088T is on the other side of the PCB.

The power switch is a minature slide switch on the side, which has an extra position for volume. This is obviously done to avoid a space consuming potentiometer. So, we have only two levels of volume; full and something a bit less.
How well does it work? Quite well actually. Performance is the same as the TDA7000 IC in terms of sensitivity and sound quality. However, the TDA7088 has the mute permanently enabled so some weaker stations that could otherwise be received with a TDA7000 or TDA7010T cannot be received on the TDA7088T. Also, the headphone lead aerial is not as efficient as a 75cm telescopic aerial so this needs to be taken into account.


Close up of the TDA7088T. The scan and reset switches are to the left.

The scanning circuit works very well, there is virtually no waiting for the radio to find the next occupied frequency. Once you have reached the 108Mc/s limit, you have to press "reset" to get back to the 88Mc/s end of the band. It does not automatically do this like a PLL type of circuit would.
These TDA7088 receivers are very common in $2 shops (look for the "scan" & "reset" buttons), and you certainly shouldn't pay more than $5 for one. Most of them use the Chinese clone of the TDA7088; the SC1088. More often than not, these cheap auto scan FM receivers also incorporate a torch. Some also incorporate an AM receiver. This invariably uses a TRF circuit based around an MK484 (ZN414). Variable condenser tuning is used with these sets, on both AM and FM. Unfortunately, AM sensitivity is poor because of the very small ferrite loopstick aerial. They're a strictly "local station only" affair as far as AM goes. Sound quality is good, however, as there is only one tuned circuit (i.e. the ferrite loopstick).


 Jaycar "Short Circuits" Personal FM Radio.
Taking over from the old Dick Smith "Funway" series of projects, Jaycar produced an equivalent series called "Short Circuits". As I understand, all the projects were designed by Silicon Chip. One of the projects is a TDA7000 receiver.

The design is elegant, fitting into a small low profile plastic case. There is no power switch as such, but instead the DC resistance of the headphones is used to bias on Q1, which is in series with the 9V supply. I'm not too keen on the out-of-phase connection of the earphones. Wired in series like that gives a peculiar unnatural sound. However, the way the socket is wired means that if a mono plug is inserted, everything works correctly. The right channel socket contact is already earthed, so inserting a mono plug does not short circuit the output as would normally happen. The power switch also works normally, as the 6.8K resistor is taken directly to earth. Despite that, the instructions imply the use of stereo earphones. The aerial is a 75cm length of wire.
Further work needs to be done on this receiver. It is not working as it should. The squelch constantly cuts in and out on anything but very strong stations.
Like a lot of TDA7000 receivers, a miniature tuning condenser is used, of the type intended for MW transistor radios. Normally the lower capacitance oscillator section is used with a series capacitor, but in this circuit the oscillator section is in series with the higher capacitance aerial section. This means the capacitor shaft is floating at RF and cannot be earthed. A plastic knob must be used. Despite this, there does not seem to be a problem with hand capacitance.
Improvements for this receiver would be to disable the squelch, rewire the earphone socket, and earth the tuning capacitor shaft.
Unfortunately, this kit has now been discontinued, but I purchased some of the last in stock just in time



Model T Car Radio.


Under dash tuning head.

For my first version of the radio used in my Model T Ford, I built a tuning head using a TDA7000. I designed and etched the PCB, based on that shown in the Philips application notes.

The radio consisted of a tuning head mounted under the dash in the usual position, and a valve amplifier with power supply under the front seat. The circuit shown is the prototype tuning head. There wasn't enough room to include the 6BA6, so it was located in the amplifier box. It was subsequently replaced with a 6CS6 which better suited the method of volume control.
The TDA7000 circuit is as per the application notes, but with a few additions to make it suitable for automotive use. The car 6V supply is regulated to 4.5V with a two transistor and two diode regulator. A 10V zener clamps any spikes on the car supply. The aerial input is protected with a neon lamp and static discharge resistor. Note the four diodes at the 6BA6 grid. These were included to protect the TDA7000 in case of an internal short in the 6BA6 or other fault which might allow high voltage to be applied to the TDA7000 audio output. The diodes clamp to 1.2V which is greater than the audio signal.
Performance of the TDA7000 in this set is the best I have yet encountered. It goes to show the correct PCB layout makes a difference.
It's very similar to the EA circuit I used with a varicap diode. So, it seems that the lower Q of the varicap does not seem to be a problem.



Performance of the TDA7000.
How does it perform? For strong local stations it works very well, with excellent sound quality. Sensitivity is good.
It has a very wide capture range due to the AFC circuit. It's quoted as +/- 300Kc/s. That can make it difficult to receive weaker stations, close in frequency to strong ones.
There can be some distortion on weak signals which can be more unpleasant than the rushing noise, when using a super-regenerative receiver on a similarly weak signal.
The TDA7000 performs no worse than most commercially made portable FM radios. In fact, it performs a lot better than some I have tried.
My recommendation is to gear down the tuning control with a reduction drive, or use a pot with varicap diode. As with all kinds of receiver, give it a decent aerial!
The noise generator capacitor and muting function are optional, and their inclusion is best decided upon for individual circumstances. The muting can make the receiver easier to tune, since only correctly tuned signals will be heard, but weak signals will not be heard at all. My preference is to disable the muting.
The receiver must be used with a properly designed PCB - the best is that shown in the Philips data and application notes.
I highly recommend this IC for where non technical users are involved and where sound quality is important. For what it is, and the simplicity of use, it makes an excellent FM receiver.


You could say the TDA7000 is my favourite IC. Here's some of my collection. The three commercially made radios use a TDA7088T clone.


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