Video amplifier Test Procedure query. 1

[drillz]

I would be very grateful for help on the test procedure for a video amplifier that my company makes.
I am sure my company will not be harmed by me anonymously posting a link to a brief-ish form of the schematic here, as its only simple inverting stages and an analog multiplier IC (AD835R).….which gives an offset to the signal as well as multiplying the signal by the “Gain/Contrast” voltage.

Here is the Video amplifier under test:- (see diag 1 Schem)

DIAGRAM 1:

This amplifier circuit has two biasing voltages….
1. The brightness (-5V to +5V) voltage
2. The “gain/contrast” (0 to 5V) voltage

-These are shown on the above (Diagram 1).
“Brightness” basically appears to affect the DC level
“Gain/Contrast” appears to affect the signal amplitude.

The “Gain/Contrast” bias voltage is fed to an AD835R multiplier whose action is as in the above (diagram 1).

I am required to test these amplifiers but I am concerned that the test procedure is sub-optimal. In particular, I am sceptical about the fact that the multiplier output is first zeroed with a pot when there is no input signal to the amplifier chain, and when the “Gain/contrast” input voltage to the multiplier is zero volts.……then later, this SAME pot is adjusted to provide a DC offset to the output signal…..
…..this seems strange, and I believe that a different pot should be used to set this DC level….

Here is the
TEST PROCEDURE:

1. Centre all pots.
2. Connect 100R load to output.
3. Switch on (power to IC’s)
4. Set “Brightness” input to 0V
5. Set “Gain/Contrast” input to 0V
6. Allow 30 mins for circuit to stabilise.
7. Adjust POT3 to give 0V at “W” output of AD835R
8. Adjust POT2 to give 0V at pin 6, IC4
9. Set “Gain/Contrast” input to 5V
10. Set “Brightness” input to 500mV
11. Apply pulse train to “SIGNAL IN”…….(see Diag 2)

DIAGRAM 2:

12. Use POT3 as coarse adjustment and POT2 as fine adjustment to set output pulse train (across 100R load) to the following diagram….(i.e. to get the signal “base-line” at 500mV)……………… (see diag 3)

DIAGRAM 3:

13. Adjust POT1 to set peak-to-peak amplitude of output pulse train to 200mV

14. Repeat 12 to 14 till output is set as follows:- (see diag 4)

DIAGRAM 4:

- - - - - - - - - - - - - - - - - - - - - -
-My problem is with step 12 in the above test procedure……I believe that ONLY POT2 should be adjusted to set the “base-line” (see diagram 3) level of the output signal to 500mV....
....Not POT3 and POT2 as required by step 12.

POT3 is first adjusted in step 7, -I feel that to re-adjust POT3 in step 12 is ruining the initial zeroing of the multiplier’s output.

I would be most grateful for any thoughts on this.

 

[VEBill]

I wouldn't worry about anyone copying the circuit. I suspect that the Chinese (for example) could create their own functionally-equivalent circuit with maybe a couple of transistors, a couple of diodes and no pots.

The first thing I'd get rid of is the 30 minute warm-up time (Step 6). If that circuit takes half an hour to stabilize, of what use is it to the end customer?

For Diagram 2, why not just adjust the signal generator down to 2 volts to start with? Is the attenuator being used to stabilize the output Z from the signal generator? Probably an unnecessary step if you can set the level anyway. and most signal generators already have an output pad built-in for the same reason.

For Diagram 4, you're basically setting the offset (500mV) and gain (200mV, or 700mV depending on how you reference it). It couldn't be simpler.

If you've got three pots to adjust two parameters, then you've got an extra pot. It doesn't look like a rough/fine setting. And you shouldn't need a fine adjustment for that simple a setting.

If the circuit is being used for normal video, and the settings are as you've described, then the circuit could probably be designed with fixed value resistors all the way through and eliminate the entire set-up procedure.

The pots will drift over time more than quality fixed resistors.

Does the circuit have to deal with video including sync pulses? What happens to the sync pulses when adjustments are made to contrast and brightness? You can adjust brightness with nothing more than a pot (if you're willing to accept some side-effects).

Your question is probably valid, but there are many bigger questions that might be even more important.

 

[melone]

When you asked the person who designed the circuit and test procedure, what did they say?

 

[logbook]

Let's start on the left.

There is a rather nasty 22pF capacitor across the inputs of the opamp. I suspect somebody was trying to reduce incoming noise, or something. Then they probably had to add the 6p8 feedback capacitor to make the amplifier stable. (The 22pF is not in a good position!) Anyway we have a (voltage) gain of 12 followed by a gain of 10 giving over 100. This looks like a clipping amplifier, where the amplifier is expected to hit its output limit (or the power rail) in normal operation.

The positive going part of the input signal is inverted twice by the output of the second amplifier, so it will clip on the positive rail (or positive amplifier limit). The low limit is then governed by the "brightness". So we have a rectangular pulse going from some adjustable level up to the positive clipping level.

Step 7 of the procedure isn't adjusting the multiplier output as such. It is providing a zero signal as an input to the next stage to zero the next stage. Pot 2 is adjusting the offset of the output amplifier. You might just as well put on a shorting link to ground at the end marked "2" of the 4K7 resistor.

I therefore don't think this is such a problem as you might at first have thought.

 

[drillz]

Thankyou all for these valuable replies.

VE1BLL....i am having doubts about the use of the attenuator myself, as well as the 30 minute "warm-up".

melone....Unfortunately the design engineer works elsewhere usually. He is very busy and i could not ask him something unless i was absolutely certain that i could suggest something which was going to be a certain improvement. Also,i am just a sub-contractor.

logbook...i am also grateful for your input here.

 

[drillz]

I must apologise......the attenuator after the signal generator cuts the input signal (to the amplifier) down to 100mV peak to peak....not 2V peak to peak as first described.

(i unfortunately got mixed up.....the 2V peak to peak signal is what is measured at the output of the signal generator after the attenuator has been connected to it....of course, with the signal generator unloaded, the tester first sets the signal to 4V peak to peak as described earlier)

Here is the attenuator (just some axial resistors).....

This amplifier is said to be for "low-level" signals.

 

[logbook]

Just as a point of drawing "etiquette" / convention, it is usual to have the input on the left and the output on the right. This is much like writing left to right on the page.

People get used to it and therefore prefer "standard" drawing practice.