FM Receiver - TDA7000 Project / Varicap Problem

Thread Starter

johncena

Joined Aug 15, 2010
32
I know that TDA7000 is obsolete nowadays, but we managed to bought somewhere near in our place. We need to construct a FM Receiver, capable of receiving signals up to 5 or more stations, ranging from 88Mhz to 108Mhz.

All of the components were bought (TDA7000, capacitors, resistors) but two of them are hard to find. The most problematic to us is the Varicap (Voltage Variable Capacitance Diode)

We used the circuit diagram here --> http://electronics-diy.com/TDA7000_FM_Receiver.php

However, the only available Varicap here in our place were the ff:

MV104 -(Dual Voltage Variable Capacitance Diode, 37-42pF (range cap))
MV2107 -Nominal Cap 22pF
MV1650 -Nominal Cap 100pF

IF we are going to use MV2107 as a substitute for MV2105 (wherein, the MV2105 has a Capacitance: 2 - 16pF ).. What are we going to do if we want to make that MV2107 suitable for the job? Should we change the Value of the capacitors nearby the varicap? or should we put another capacitor in series/parallel? , modify the coil? or change the value of resistor? in order for us to produce a smooth capacitance increments for tuning.
 

bertus

Joined Apr 5, 2008
21,362
Hello,

Did you see this in the partlist?

MV2105 is a variable capacitance diode that is used as a frequency tuning component in FM Radios, FM Transmitters, TVs, VHF Receivers, etc. MV2105 varicap may be used as a replacement for BB105 and BB205 varicap diodes.

Varactor Specifications:
Capacitance: 2 - 16pF
Voltage: 1 - 30V
Case: T0-92
Bertus
 

bertus

Joined Apr 5, 2008
21,362
Hello,

The other varicap can be used.
Only the coils need to be smaller in that case.
(spreading the coil may already be enough).

Bertus
 

Thread Starter

johncena

Joined Aug 15, 2010
32
Hello,

The other varicap can be used.
Only the coils need to be smaller in that case.
(spreading the coil may already be enough).

Bertus
thanks again Bertus, that clear things up :D

i'll update this post as soon as i build the circuit.
 

Thread Starter

johncena

Joined Aug 15, 2010
32
ok another problem...

MV1650 and MV104 Varicap are the only available components here. (mV2107 out of stock >.<)

If i'm going to use the MV1650 (min of 90pF to max of 110pF). what will i do to get that 88Mhz to 108Mhz frequency?

An example was using 106nH inductor and a 41.4pF to 20.5pF Varicap will yield an exact 76 to 108Mhz, which covers all the FM frequency band.

if i will use MV1650 (neglecting MV104 since it has 37pf - 42pf), is there anyway to achieve that Fm band?
 

bertus

Joined Apr 5, 2008
21,362
Hello,

You could try to change the 1 nF capacitor to a lower value like 100 pF and see what happens.
The total value will be smaller than in case of the 1 nF capacitor. (series connection of capacitors).

Bertus
 

Thread Starter

johncena

Joined Aug 15, 2010
32
hmm..

90pf in series with 100pF = 47.37pf
and
110pf in series with 100pf = 52.38pf

and 47.37pf with 70nH will result in 87.7Mhz
and 52.38pf with 70nH will result in 83.4Mhz
 

bertus

Joined Apr 5, 2008
21,362
Hello,

You should try to find a varicap with a comparable Cmax / Cmin ratio.
The ratio of the MV1650 is to small to cover the FM band.

What other varicaps can be found at your place?

Bertus
 

Thread Starter

johncena

Joined Aug 15, 2010
32
only MV104 and MV1650 are available.


I do have old non working TV here, maybe i can find Varicap diodes there..
I'll check it later..
 
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