FM Demodulator with Tv Tuner

[Masood_786]

Hi All,

I am interested in building FM receiver scanner based on Tv tuner ( varactor controlled TDq-37-5v 5v based tuner). I have implemented the circuit as mentioned in the website
uzzors2k.4hv.org/index.php?page=tvtuner
I am not able to receive anything, in theory the 38.9Mhz signal from tuner goes to saw filter before demodulation. I have connected cable tv input to Tv tuner but no luck. One problem i figured out that My LM7805 not generating 5v but 4v as required by my tuner for band selection. Is accurate power supply necessary for Tuner.
Some info on tuner
Name: TDq-37-5v
BH: 0/5 volts (0 for no selection and 5v for selection)
BL: 0/5 volts
BU: 0/5 volts
BM: 5v
AGC:4volts (fixed)
Vt(tunning):0-30v

 

[betwixt]

I do not understand why you are getting 4V from a 5V regulator, and yes, it needs to be fairly accurate.
Either:
1. the regulator is faulty
2. it is overloaded, this is unlikely but if it is it will be getting very hot
3. you do not have the necessary 8V or more at it's input pin. The extra 3V is needed for the 7805 to function properly.

Note also that the circuit uses 12V for tuning but you are saying it needs 30V. If it needs 30 but you only tune with 0 to 12V, you will not reach the higher frequencies in the band.

Brian.

 

[Masood_786]

Hi Brian
Thanks for your kind reply,
Actually at the Link, uzzors2k.4hv.org/index.php?page=tvtuner, it is 12v tuner but mine is 0-30v tuner. I was not able to get 12v tuner as mentioned in the link above and you are correct in saying that i would not get full coverage. I actually don't have 30v supply so i have to compromise for the remaining frequencies in the band. I am planning to use computer supply for providing +5v(regulated for band selection and BM pins) and +12v(regulated) for tuning (not full coverage). I think PC supply would be helpful. Any ideas ?

Masood

 

[davenn]

yes a 5V suply from a puter supply would prob be ok, but be aware, depending on the quality of the PSU, you may find it a bit noisey ( puter supplies are prone to producing a bit of RF noise.)

As the others said you need at least ~ 3V more into the 5V reg to be able to get a stable 5V out so practically speaking a minimum input of ~ 8V.

ok some questions for you......

what FM freqs are you trying to receiver with the tuner ?
that will determine if the tuner will even be able to cover those freq's

do you have a signal generator that covers the freq range you want the tuner to receive across ?
The 38.9 MHz signal out of the tuner is the I.F. freq, any incoming signal gets mixed with the local osc in the tuner to result in that IF freq.

Do you have a 50 or a 100MHz oscilloscope ?
it would be good enough to resolve the 38.9MHz signal oout of the tuner to see if it was working before that signal was injected into the SAW filter and the rest of the cct.

cheers
Dave

 

[betwixt]

As davenn stated, PC supplies are designed to produce as much power as possible at the lowest price. Keeping the output clean of noise is not critical in a PC so expect some interference from it. You might also hit a problem with some PSUs because they rely on a minimum load current to prevent them shutting down.

The schematic is 'odd' in that it uses a domestic AM/FM radio IC to do the demodulating but the LO is set to 27.3MHz, this does not seem to mix with the tuner IF to produce 10.7MHz so I assume it uses a peak in the SAW filter response rather than being centered in it. Without further filtering it will have a very wide bandwidth before the IF mixer so you might get unwanted blocking effects or overload if you have a strong station nearby.

Brian.

 

[Masood_786]

Thanks Dave & Brian for the reply

@Dave
what FM freqs are you trying to receiver with the tuner ?
I have cable tv signal at input so at first i will demodulate Tv Channels(UHF VHF FM mode) sound, if i am successful then i will demodulate Air Traffic communication transmission (AM mode)

do you have a signal generator that covers the freq range you want the tuner to receive across ?
I am using cable tv as input

Do you have a 50 or a 100MHz oscilloscope ?
I have frequency counter upto 100 Mhz

@Brian
In the web site i have mentioned, there signal is mixed with 28.2 Mhz signal, it's written in theory see last paragraph, in schematic it's shown 27.3Mhz (i think it's wrong)

Now as you both have mentioned i will buy some battery or adapter . I think they will not be as noisy as PSU and may be i have luck this time.

Masood

 

[betwixt]

You might have trouble with AM reception. The AM section of most of this type of IC is only good to a few MHz. Toshiba do not specify the maximum frequencies but I would guess it is nowhere near as high as the tuner IF. You might also have trouble with frequency drift. The tuner does not use a PLL to stabilize the frequency so it will be very dependent on voltages and temperature. For TV which has a wide frequency spectrum this is not a problem but when you narrow the bandwidth down from about 6MHz to 10KHz or so, any drift will be very noticeable.

Brian.

 

[davenn]

ya beat me to it Brian

agree totally, Even the FM Broadcast band (88-108MHz) if the tuner will cover it will probably be a problem for the same reason, even tho they have a somewhat wider bandwidth than AM signals

ATC signals ~ 118 - 125MHz are in between the Vl and Vh bands and depending on the particular tuners' capabilities may not be reachable anyway

 

[Masood_786]

Thanks Dave and Brian for your kind suggestions/replies. i will inform you soon when i am successful in receiving any of the channels.

Masood

 

[Masood_786]

Hi,
I have come into the problem may be this is the cause of Tv Tuner Malfunctioning that when i apply +5v to tuner for band selection(BH, BL, BU and BM pins) then the voltage drops to 4.1v , i think that's the reason i am not getting IF signal out from Tuner, My tuner is TDQ-37-5v but i dont know how much current it requires, can't get it's datasheet, 7805 can deliver maximun 1A. Does any one know the current rating for tv tuners (Is it standard ? mine is analog not digital)
Also can anybody confirm that 4.1v instead of 5v is ok (in case of analog tv tuner) for band selection pins , i don't know if these are TTL or CMOS, if it's TTL then 4.1v should be ok but in my case i ca't get any output from tuner also for AGC(4v recommended) i have attached potentiometer for adjustment but no luck.
A prompt reply would be helpful

Masood

 

[vfone]

The TV tuner shouldn't take more than 100mA with any of bands switched ON. Digital tuners with internal prescalers may take little bit more current than analog ones. 5V tuners may not work at 4.1V.
7805 can deliver 1A only if is supplied with a source that can get 1A. Check the voltage drop at the input of 7805 when output is loaded.
At the beginning I would check if the circuit using TA2003P is working as a stand alone circuit (you need a modulated signal generator). If this is not working, you cannot verify if the tuner is ok.
Check the link below how to connect the AGC and the AFC pins of the tuner to make it work.
https://hem.passagen.se/communication/pic/scemt.jpg

 

[betwixt]

Usually in tuners of that kind, the bands selection voltage only goes to a single transistor local oscillator and some band switching (PIN) diodes. the current should be quite low and no more than a few mA. As vfone suggests, check you have enough voltage ahead of the regulator for it to maintain stabilization. I would have thought that if the tuner could pull a 7805 down to 4V you would see smoke coming from it!

Brian.