Help with and INTENSITY "marker adder" circuit for Sweep Gen and Scope

[gmcjetpilot]

I am trying to make a circuit that will take the "Demodulated" signal from a radio that is being sweep-ed to align the IF's .... the resulting trace on the scope is the classic freq response. The idea is to get the peak at the CENTER FREQ of the IF .... typically 455Khz for AM radio and 10.7Mhz for FM.... To identify this center freq you need a MARKER, some kind of indicator along the trace at that point. OK. The old way is to add in the marker or another RF signal into the radio and it BEATS against the sweep as it goes buy. The down side is the marker goes through the radio and affects the results. Also the marker appears on the scope as rather large and wide..... NO GOOD.

The next step is to add the marker after the signal has gone through the radio. This is called "Post-Injection". The marker does not affect the radio or distort the trace. This requires some mixing which I have in another thread, based on some old Tube designs I might try to make a Solid State Version.
https://www.electro-tech-online.com/threads/sweep-marker-adder-help-with-design-rca-wr-70a-tube-adder-to-solid-state.105309/

However I think the elegant way is to have the RF sweep (in parallel before it goes into the radio) mix with the marker signal and compare them. When they are at the same Freq it triggers a signal, which can be a pulse. I have not tried a Z input to the scope, but I think the function of the Z input is to change the intensity of the trace, HIGHLIGHT the trace with a change in voltage to the External Trigger. So the sweep trigger from the sweep Gen can go through the MARKER ADDER and momentarily increase the voltage to the scopes as it SWEEPs, making that highlight at the right time. This way there is NO mixing of freqs or mixing of the "Detected" Freq response of the radio with the marker..

This is the circuit as part of a total DIY sweep+marker using a MAX038 chip a Gentleman made. I want to try to graph the Marker part from it but not sure it will work for reasons I give below. It comes from this site and the marker description is down the page 2/3rds.
http://www.cappels.org/dproj/functsweep/functionswp.html

**broken link removed**

The CD4051BE can be eliminated and just selects one of two markers. The CD4051BE is apparently just making pulse at different periods. I THINK DON'T KNOW the TL064 is a OP AMP buffer? I guess it takes the marker input from the CD4051BE. I am not sure this is where I can substituted a RF signal gen. This is all voltages and timing.... how to relate to freq is the question. Than there are two LM383 "Comparator's" of the "saw tooth" and marker voltage" The result is a pulse? Not sure. The two 2N4401 transistors are a mystery to me. After that it will need some more circuits to add a pulse to the sweep trigger from the sweep generator to the scopes Ext trigger (and Z intensity) input.

Not sure about marker voltage out and blanking circuit. The saw tooth is a problem. It looks like he is dealing with timed pulses and saw tooth voltage. I am dealing with RF Freq variation (sweep) with constant voltage and and another RF signal gen for a marker. Not sure these comparators will READ or compare Freq? Basically I need a way to read and compare Freq, to trigger a little pulse into the scope trigger input to increase intensity.

The SWEEP Gen I have is the HP 8601A. The manual is here:
https://www.electro-tech-online.com/custompdfs/2010/03/HP208601A.pdf

So the idea is another connection will go to this box, the trigger from sweep to scope will pass through the ADDER. The trigger signal I am not sure of. I am not sure about all the connections. However I am not married to this, just think this might be in the right direction.

 

[ericgibbs]

hi,
A method we used to use was to use the 'Z' modulation input of the scope, to brighten the trace 'spot' at the 455kHz or10.7mHz frequency.

 

[gmcjetpilot]

hi,
A method we used to use was to use the 'Z' modulation input of the scope, to brighten the trace 'spot' at the 455kHz or10.7mHz frequency.

Thank you dear Sir. That confirms it's a valid approach but the question is HOW? The problem is the devices to do this are not available on the used market or too expensive. HP 8600A was a "digital marker" but they are impossible to get. Also I suspect as with HP it is full of proprietary Chips. With off the shelf IC chips today, I think what took a box of parts can be done easily. I just don't know how to do it. I just don't have the design background to invent one from scratch....

The first problem to solve is Read the Sweep Freq and Read the Marker Freq..... when the EQUAL I get a trigger. That is the hard part.

From this SWEEP FREQ=MARKER FREQ trigger, a small voltage device can take sweep trigger from the sweep generator and add a little BLIP to the trace. The sweep is a modified saw tooth from 0 to 7 volts with a momentary retrace at zero...

I don't know how Z works on my TEK 2225 scope. It does not have a separate Z input. The Ext Trigger input has a switch that selects Line or Ext Trigger or Z .....

 

[Reloadron]

I remember doing stuff like this years ago. Many of the older Tektronix scopes like the 545 series were great for this stuff. They had features like dual sweep that allowed A intensified by B.

Essentially you use a sawtooth to externally sweep the scope. Let's keep it simple. Let's say a sawtooth has a rise time of 1 mS. That is zero to the top. Assume 10 horizontal divisions on the scope and that would be .1 mS / Div.

Now let's also assume a pulse that has the same duration as the sawtooth wave. Meaning a pulse width of 1 mS. That pulse is the Z axis information. The trailing edge of the sawtooth and the falling edge of the pulse are blanking. This way the CRT is shut off during the retrace interval.

If we could place a pulse on top of our pulse during the time of that pulse our intensity would increase on the CRT. If we could move that pulse back abd forth accross the main pulse top we would get an intensified portion of our sweep for the duration of that pulse at a point in time where the pulse was on top of the Z axis pulse.

I see some of that in the drawing you attached. Unfortunately I am at work right now but when I get home and this weekend I can try to look more into it. What I mention here is "iffy" and I am not sure of as again it has been years since I worked anything RF or used sweep generators.

Ron

 

[ericgibbs]

hi,
I have searched all my old documents I cannot find the marker circuit I mentioned, I must have left it on the Ark.

Have a look at this link:

functionswp

 

[Reloadron]

Pretty slick circuit that should work. Now that I am home from work (out of the rabbit hole and back to reality) and had a nice cold beer the circuit posted makes good sense.

Starting at the beginning (a good place to start) the CD4051 is just an 8 channel multiplexer with only two channels used. The chip is an analog mux so whatever goes in is what comes out when the input channel is called. The Marker 1 and Marker 2 signals are set between -5 and +5 volts via pots R43 and R44. Those analog values are placed on pins 13 (Channel 0) and pin 14 (Channel 2) of the 4051. Pin 11 is the A input for select and a low on pin 11 will select channel 1 out and nothing (open) on pin 11 will select the channel 0 output. The selected channel will output on pin 3 (output) as an analog value depending on the pot settings.

Looks like R45 and R46 form a voltage divider and the analog signal is passed on to U1 a TL064 which is no more than a buffer. No clue where the rest of U1 actually is?

Now U4 the LM393 is configured as a Window Comparator. **broken link removed** if you scroll down shows the 393 as a window comparator. Note how that 393 is configured and how at this point the sawtooth (actual sweep) is applied.

Depending on the set marker voltage as the sawtooth ramps positive the window will open and the output of the LM393 will produce a pulse for the duration of the window.

The marker out goes to the scope Z axis and the sawtooth goes to the scope external horizontal input. Assume the sawtooth is available. That is how I think (think being the operative word) works. Time for another beer!

@ Eric, loved the part about the Ark!

Ron

 

[gmcjetpilot]

Pretty slick circuit that should work. Now that I am home from work (out of the rabbit hole and back to reality) and had a nice cold beer the circuit posted makes good sense.

Thanks this is voltage as I thought. Thanks for evaluation of that circuit. It is dealing with voltages. Which is easier. I want to work with RF Freq.

This is what I am thinking..... This would be less than perfect but the key is taking TWO analog RF signals, one fixed one sweep and when they are the same (harmonics or beat together) it would trigger a pulse of 5 volts or more to the Z or intensity input of the scope, to highlight the trace at that specific time.

**broken link removed**