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.
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
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.
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.
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.
TDA7000 receiver works from 240V mains.
Circuit of the mains operated 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.
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).
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
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.
You could say the TDA7000 is my favourite IC. Here's some of my collection. The three commercially made radios use a TDA7088T clone.