EMRFD Message Archive 9423
Message Date From Subject 9423 2013-11-26 09:46:16 johnjdignan Can any body help me with a Varactor Circuit? 9429 2013-11-26 16:52:13 Ashhar Farhan Re: Can any body help me with a Varactor Circuit?
The link took me to a sign-up page. However, here goes...
We must absolutely step out of our comfort zone. The way to get
comfort back is by measuring.
I suppose, given your info that you will use a separate carrier
oscillator. You have a choice of either modulating the local
oscillator or the carrier oscillator. However given that you will want
a 6KHz deviation, getting that out from the 10.7 MHz crystal with
linearity is going to be tricky. You could use a superVXO
configuration or simply do away with the crystal oscillator and use a
VFO. A 1 KHz drift is almost unnoticable on nbfm.
Once you decide which oscillator to modulate, you can measure the
swing by simply tuning the oscillator with the varactor diode. Measure
the voltage at 1KHz points and note them down. Then plot them to find
a linear spot where the freq vs voltage is linear. This plot will also
tell you how much swing is needed from the modulator.
Depending upon the L to C ratio of the oscillator, the voltage swing
required can be very little, less than a volt.
Now, you can get back to your modulator, add clipping to the 1V output
with two diodes, add a. 'Volume control' trim-pot and feed the audio
to your VCO.
Once you have FM modulating, you will want to add filtering to
pre-emphasis as well as remove the audio harmonics generated by the
diode clippers.
Really, the challenege of the fm modulators is on the audio side. To
keep the deviation within the narrow-band iis solved audio clipping in
a quick and dirty way. The correct procedure would be to use ALC with
low pass filtering. I haven't tried this, so I am not sure. What I did
try though is to use a Yamaha home recording mixer that has a built-in
audio compressor with a NBFM tx. The clarity was amazing. I ddint have
anyway of quantifying what I heard though.
There are ways to check the deviation very simply by injecting
specific tones and doing some maths with bessel functions. I read it
in a 70s ARRL Handbook but I forget the details now The latest
Hanbook, though larger in size seems to have less and less information
and more and more ads.
- f
On 11/26/13, johnjdignan@yahoo.com <johnjdignan@yahoo.com> wrote:
> Hi All,
> Due to a change of VFO I now need to find an alternative way to provide FM
> on my BitX. I've found a circuit below which I'd like to use, but it has no
> component values.. It's figure 3 here:-
> http://rbsfm.org/am/index.php?option=com_content&task=view&id=29&Itemid=29
> http://rbsfm.org/am/index.php?option=com_content&task=view&id=29&Itemid=29
>
>
>
> My carrier osc. is at 10.7Mhz outputting around 4dbm and the audio is
> around 1 volt Pk2Pk the output of this circuit will go to one of Wes's
> impedance insensitive Bi-di amps@ 50 ohms. I know EMRFD is not big on the
> subject of FM, but I'm on here and thought I'd ask.
>
>
> BTW I'm just an experimenter not an expert-still finding new stuff to
> learn, I'm a little out of my 'comfort zone' with this, so please make any
> response not too.overtly technical and go easy on the math!
> Best regards to you all!
> John M0JJD
>
--
Sent from my mobile device9446 2013-11-29 06:11:46 johnjdignan Re: Can any body help me with a Varactor Circuit? Thanks Farhan,
I have been thinking about what you said about getting the audio right as being the most difficult task, You mentioned getting good results with a speech processor. This jogged my memory back to a radio rally in May where I found two complete and fully assembled little circuit boards with a data sheet that described them as "FM board speech processor and FM de-modulator"
For 2 pounds I bought them both and I'd forgot all about them until i read your post, I've just dug them out of the junk pile and think they might come in handy. The data sheet is quite good comprising of a schematic,component list, parts layout and interestingly, a performance curve.
This shows a peak at 300hz (6db octave) which then gradually slopes down at around 25 degrees below the horizontal to 2800HZ where it then rapidly rolls off down to the 18 db octave.
(If you like, I'll scan it in and send you a copy-- if you give me an email address I can send attachments to).
Fortunately using the demodulator section is optional and as it requires an IF of 455khz I'm not inclined to use it. The speech processor section starts with a PTT circiut and the goes through four consecutive op amps (the first being for mic gain) and so on. I'm guessing that the pair of diodes in the middle which are anode to anode split by a resistor are involved in some form of compression functi
9448 2013-11-29 08:35:29 Ashhar Farhan Re: Can any body help me with a Varactor Circuit?