look at this cheap network analyzer - -mainly good for audio

[Opcom]

this is very interesting for those experimenting with audio circuits such as filters and amplifiers.. It could replace or augment several instruments. It seems like the actual filter curves could easily be measured as well as the total behavior from mike jack to modulated B+.

http://www.syscompdesign.com/circuitgear.htm

[AB2EZ]

It's really amazing how rapidly the capabilities are rising and the prices are falling on this sort of stuff.

As just one example:

Combine this with a simple envelope detector and (if you work with the software) you can build a modulation monitor that will produce whatever type(s) of readout(s) you like; and which will automatically adjust its carrier level setting. It will even generate audio test waveforms for driving your transmitter while you are monitoring the modulation. Once someone develops the application software... it can be distributed to whoever wants it, at no incremental cost.

This stuff is really great for educational applications... allowing you to provide each student with his or her own "personal laboratory" (to take home) for a price that is comparable to that of one new textbook.

Stu

[Opcom]

Noted they use TKL or tickle. the scripting language. I did not think much of it until I saw an entire POTS and ADSL switch development lab being run with it. Including the "telco" system as a whole, as sold and installed having thousands of lines. I was amazed.

[K6JEK]

Noted they use TKL or tickle. the scripting language. I did not think much of it until I saw an entire POTS and ADSL switch development lab being run with it. Including the "telco" system as a whole, as sold and installed having thousands of lines. I was amazed.

There is nothing like an inventor and evangelist who never gives up and finds a way to pay the rent during the lean times.  In the case of Tcl/Tk that's John Ousterhout.  Bert Sutherland brought him to Sun in the 90's and built a little team around him for Tcl/Tk. But then James Gosling came up with Java and stole all the oxygen out of the room.  James  took off with Java and brought everything in the company that wasn't nailed down with him. John struck out on his own and somehow kept it going.  More power to him.

[AB2EZ]

I purchased one of these, and I received it about an hour ago.

I installed the application software and the HW driver on an XP laptop (a 6 year old Dell 600m: 1.3 GHz Centrino processor with 1GB of RAM.

It works like a charm.

I'm looking at the following:

Trace A: the 455kHz output of a receiver
Trace B: the detected envelope of trace A (using a precision rectifier and the low pass filter).

Trace B is superimposed on the top of trace A

This looks just as good as it does on my regular Tektronix digital scope. It is easy to trigger off of trace B, and to capture and hold (until the next rigger) the modulated envelope.

Even as a stock unit, it calculates and displays the average value of a trace, the maximum value and the minimum value. This is, of course, everything you need to calculate the positive and negative modulation indices. With a little extra software, one could display what any modulation monitor displays.

I haven't tried any of the other capabilities yet.

Stu

[AB2EZ]

Here is a screen shot (a feature of the application) of this little oscilloscope in action:

Red trace: the 455kHz output of my R30a receiver
Blue trace: the peak-detected 455kHz output of my R30a receiver (using a precision rectifier and a low pass filter to process the 455kHz output signal)

The gain of the peak detector is adjusted, so that the blue trace is on top of the red trace.

The triggering is off of the blue trace, and the triggering level is set near the baseline... so, with a typical AM signal tuned in on the receiver, the triggering occurs about once a second (or less). Each time the envelope of the signal drops low enough to trigger the oscilloscope (around 95% negative modulation), a trace is painted on the display... and stays there until the next triggering event. 

[Opcom]

That is a very mice detector there. It's amazing how much that little box does. It's like having a 7 FT rack of equipment that fits in one's pocket.

[AB2EZ]

Attached below is the screen capture (.jpg) of this device being used in its network analyzer mode:

The device is producing a sine wave at its bnc audio output port, whose frequency is swept (slowly) from 100 Hz to 10,000 Hz. These limits were set by me, using the software-based sliders visible in the attachment. [In other tests, I started the sweep at 10Hz]

This swept sine wave is used to drive the line input of my Class E  transmitter (which employs a Class H modulator).

The modulated rf output of my Class E transmitter passes through an "off-air pickup" consisting of an RF sniffer / envelope detector (a resistive voltage divider across the output cable, feeding a diode-based envelope detector). The off-air pick-up produces a voltage that follows the modulated envelope. The output of the diode-based envelope detector in the off-air pickup is then passed through an audio buffer amplifier (an op-amp)

The buffer amplifier output is connected to bnc input port B of the device.

Note that the measured modulation index is down about 3dB at 9 kHz. I'm currently doing some detective work to determine the reason for this rolloff... even though the measured 3dB bandwidth (9 KHz) of the transmitter is more than adequate. 

[Steve - WB3HUZ]

Nice! I like that minimal phase shift through most of the passband too. None at the low end!