Problem in tunning SSB tx/Rx

[devucool]

mc1496 bfo

Hello friends,

I am facing problem in tunning SSB transmitter & receiver at 14 Mhz.

I am using crystal ladder filter of 4 Mhz for filtering one side band. I want bandwidth of 3 Khz for transmitting voice.

I am not getting exact 4 Mhz crystal for that so this may be problem.

Plz help me it is urgent.

I attached circuit diagram here. Plz give alternate method of filtering or suggest suitable method .

 

[vfone]

russian ssb transmitter project

Ladder Crystal Filter should work fine for your application.
Here is a nice document how to build these kinds of filters:

http://www.geocities.com/leon_heller/ladder.html

Also you need to put a trim cap in series with BFO crystal, to allow tuning up its frequency. The series inductor L4 is pulling down the BFO frequency.
From my experience NE612 (and also the old NE602) do not provide very good carrier rejection as a DSB modulator. The reason is one of the internal buffers that is not biased enough. As an RF mixer works well.
I would recommend for DSB modulator the old MC1496.

 

[devucool]

mc1496 mixer

Thanx for your reply. Now i removed all BFO components . Instead i put crystal oscillator with trim capacitor in series.

But problem still lies in removing one side band. Output is coming upto filter but within filter ouput reduces & not getting exactly side band. Getting distorted output.

Can u provide me suggestion regarding removing one side band. How can i remove it without disturbing one side band.

For MC1496 i need to change entire pcb. PLz provide me solution of this project.

 

[vfone]

ssb ne612

Distorted output you can get if the filter have big ripple or if you over-modulate the DSB modulator. Other reason could be as I said, the low performances of NE612 as a DSB modulator.
Filter’s low rejection of the side band wouldn’t distort the signal (just lose some RF energy in the unwanted side band). Definitely you need a minimum of test equipment to verify the shape of the filter.
Using a scope, check the DSB signal (pin 4 of U1) to look as in the picture attached.
Tuning the BFO frequency and using a spectrum analyzer or a selective receiver on pin 4 of U2, verify the rejection of the unwanted sideband (SSB signal).
Ideal rejections for carrier and for unwanted sideband should be better than 40dB.
MC1496 can get up to 60dB carrier suppression when NE612 can get maximum 30dB.
https://www.onsemi.com/pub/Collateral/MC1496-D.PDF

 

[Borber]

ssb tx

You have not given us crystal frequencies for rx and tx oscillators so as pass band of crystal filters.

 

[docel]

dsb ne612

hi devucool,
1. You may be misinterpreting the output: how do you mean 'distorted'? Because DSB IS after all a distorted output. Also Carrier attenuation is not absolute , it is only suppressed by some decibels(db).
2. With 'amateur' test and assembly, you may look for some 20db suppression of the carrier.
3. Another point to note carefully is Carrier Oscillator leakage through other parts of the circuit. This is a major issue in Bal.Mods. Proper screening should be experimented.
4. The level of carrier should be adjusted for minimum distortion and maximum suppression.
5. The Input/ Output impedances/termination should be close to the calculated values. THIS IS VERY IMPORTANT.
There is nothing wrong with NE612 and i have achieved 30 db suppression quite easily. This should be the maximum, as pointed by vfone. Crystal Filters of 8Poles will give you abt 60db suppression.

:arrow: You can listen to the output signal using a broadcast radio at about 2ft from the modulator and tuned to the Xtal frequency. If you hear 'donald duck' distortion audio, your Bal, Mod is fine. Tune for maximum 'donald duck voice'. If side-band is generated , you must retune the radio about 2-3kc up or down from the frequency where you hear the unbalanced AM modulation. This depends on the Carrier oscillator grequency (LSB or USB)
This is the cheapest and best test and alignment tool while setting up modulators.

 

[devucool]

ne612 crystal oscillator

Hey thanx a lot.

All crystal oscillators having 4 Mhz.

First voice signal is modulated at 4 Mhz then one of side band is removed then it is upconverted to 4 Mhz signal.
That is output frequency will be (8 Mhz + voice s/g(0-4khz)).

I am using PTH board so that leakage will be reduced.

I am not able to hear donald duck like voice.

And main problem is that our college dont have oscilloscope. So how can i test? Or else i go to company & check out on oscillscope.

THanx for helpming me.

 

[Borber]

bfo ne612

One thing you must checque first. Does the local oscillator Y1 in tx have about 300Hz lower frequency than the lower band limit of crystal filter is. That is valid for USB. Then you can assume that generation of ssb is correct.

 

[vfone]

ne612 bfo level

DSB (Double Side Band) is not a distorted signal.
Both side bands carry the same information and each of them could be suppressed, without distorting the information.

To improve the performances of NE612 (as a DSB modulator), you can add a 50 K pot (wiper to ground) across pins 1 and 2, and balance it out a little better.

But the lack of that internal bias that I mentioned early it will not give you that nice and “round” audio modulation, target of every ham radio.

 

[docel]

russian ssb - diagram

HI DEVUCOOL!
1. "Donald-duck voice" is how SSB sounds at the AM receiver, with out BFO injection.
It is a fovourite term used by HAM radio homebrewers ( atleast used to be).
2. The AM receiver and a loose coupled outboard BFO is sufficient for carrier Nulling and other modulator alignment. I have never used an oscilloscope to align the modulator till now!
3. vfone, You are right : DSB is not a 'distorted' signal per se. The description is just symbolical as calling any SSB signal as "distorted with respect to an AM signal":!:
4. Your suggestion of 50k pot is perfect. A multi=turn preset will provide better control. But there is a serious shielding problem.

5. devucool: PTH boards are not leak proof. I have seen your lay-out. You need to attach a ground plane below the modulator area ( solder side) and connect it to ground, like a lid. Build a box around the filter, abt 2mm more than the xtls of the filter. A single side PCB is ideal. These techniques will help to keep the carrier out of the modulator other than the signal input point.
:arrow: Difficult to understand, but it is easy to "see" this: A radio receiver kept at a distance ( say 1 feet away) and with no electrical connections will receive the oscillator signals. This is proof that the signal is :arrow:radiating, too much, not shielded sufficiently. If it is picked up from 1feet away, how about some centimeters??
6. You say 4Mhz + $mhz= 8MHZ o/p....
This is not favourable:!::!: Change the output to 9Mhz or somethging that is not a multiple of 4Mhz ( XTL or Car. Freq). This will avoid superfluous signal injections.

:arrow: Just remember: every oscillator is a transmitter and will radiate, no matter what the power level:!::!::!:: This is the biggest problem of RF signals cry:

 

[devucool]

ssb mixer when usb turn lsb

Hey vfone, docel thanx for ur reply.

I will do what u said regarding 50 k pot. And i will change to upconverted frequency at 10 Mhz.

Yea docel problem is that every crystal is radiating. even that crystals of bandpass filter is also radiating . HOw to do shielding?

I will do what u all told & then will post reply.

Bye for now

 

[docel]

ssb modulator ne612

Xtal: Solder a wire from Gnd to the Xtal body.

 

[artem]

ne612 ham

If you dont have a scope - you can not be sure where the problem is . I think troubleshooting has to be done starting from audio , 1st mixer, filter, 2nd mixer , output stage .
What is the receiver you use to test for SSB? Is it under tuning as well ?

Did you check you audio amplifier output ?
Do you have ssb capable working for 100 % receiver covering 3-15 MHz?
if so check signal after first mixer output pin4 before filter by coupling receiver with mixer. SSB capable receiver should able to receive DSB perfectly and suppress unwanted side . If you dont get any ducky sound from receiver - either first mixer or audio path is faulty.
Then test rest of the part

You can also check the first mixer and filter by applying same 4 MHz to second mixer and checking resulting demodulated AUDIO on pin 4 of second NE612.

Make a HF detector o zero bias diodes and check voltages on oscillator and mixer outputs.