Using the HP3586 Selective Level Meter With ARGO
December 7, 2002
Selective Level Voltmeters are very useful as LF receivers, but how do you set them up to operate with ARGO?
This article will help you get the most from your HP3586 SLM and ARGO.
Much of this information will also apply to other types of Selective Level Meters and receivers.
When I first started working on LF,
I was unsure of the best way to receive the signals. Since I have several good HF receivers, I naturally thought that using a front-end converter would be a pretty good way to go. So, I went back into the recesses of my storage closet, and after some serious digging around, I finally found my old Heath HD-1420 VLF converter.
As a VLF converter, it turned out to be not too bad, but, unfortunately, it suffers somewhat from RF leak-through on the output. I was unable to completely eliminate this problem after quite a bit of work, so that eliminated the HD-1420.
I decided that as receivers go, this was a Yugo. I definitely needed something better.
I decided that I would have to start looking for a better receiver.
I thought that I might have found it when I tried my trusty Sony ICF-1020. But, alas, it was not to be, for although the Sony covered the LF band quite well and was even more sensitive than the Heath converter, it also was quite temperature sensitive, and would drift up and down as the room temperature changed. This problem eventually drove me more than just slightly crazy when running ARGO, since I could not "zero in" on a weak signal without having the signal drift off the screen in a fairly short time.
This one turned out to be a Honda. There had to be something better out there. Hmm... maybe I should take a look at eBay. ...and so...
I managed to obtain
a Cushman CE-21A Selective Level Meter (SLM) for a very low bid, and found that it did, indeed, perform quite well as a LOWFER receiver.
Although this was several orders of magnitude better than the Sony, it was not perfect. Being a typical engineering type, I tend to use stuff beyond it's design limits. I'm sure that the Cushman's manufacturer would have been proud of the accuracy and stability of the CE-21A, but I was always trying to get just a little more from it than it wanted to do.
Although it had no problem holding stability to a cycle or less per day, it too, was just a bit temperature sensitive, especially when I was trying to look at that last tenth of a cycle for hours on end. I eventually wrapped it in a thick bath towel to hold the temperature of the unit more constant. That pretty well did the trick. On the plus side, the sensitivity and garbage rejection was excellent, and I was hard pressed to ask for a more sensitive receiver. But, still, I wanted more...
I guess I had a Chevy or maybe a Ford in this one.*
...back to eBay.
After watching that great Hamfest in Cyberspace,
eBay for a few more months, I finally happened upon a really good SLM, a HP-3586-C. Covering the frequency range of 50 Hz (yes, Hz. not KHz) to 32.5 MHz in 1/10 Hz increments, it looked like a great unit to use as a LOWFER receiver.
I put in a modest bid on it, and much to my amazement, I won! I was quite surprised, since I had expected it to go for about two to three times what I bid, but who was I to argue with success!!
A week or so later, and there it was, all 40+ pounds of precision instrument, built to typical tank-like HP standards, just waiting for those elusive LOWFER signals. After checking it out and calibrating it (lots of fun without a manual) it was time to get down to business.
The frequency stability and accuracy of this unit is impressive, and I think it will be quite difficult to find anything much better for LOWFER work
This time, I think I have gotten the Rolls-Royce. <G>
Some comments on ARGO
My software program of choice for pulling weak LOWFER signals out of the noise is ARGO. I'm sure all the members of the LOWFER community will give a hearty round of applause and a rousing cheer to I2PHD and IK2CZL for their contribution to the state-of-the-art by writing ARGO.
For ARGO or any other weak signal detection program to function properly, the entire system must be set up correctly. Although almost any sort of lash-up will work with ARGO to some degree, best performance may be obtained if you follow a systematic approach to building your weak signal detection system. No matter what the receiver you are using is, the same basic requirements for the computer system will apply.
To begin with, there are substantial differences between computer sound cards. Strangely enough, the most expensive ones are not always the best, so, if you have an old sound card and computer, you may very well get excellent performance from them
I have found the following procedure to work well for me in setting up the system.
After installing the sound card and making sure that it is working, install ARGO and start ARGO running. Select the FULL BAND VIEW mode if ARGO does not start in that mode. Make sure the Visual Gain AGC is on.
Now, open up the Windows Volume Control box and turn off all the record inputs except for the LINE IN setting. (You will connect the receivers audio output to the LINE IN jack on the sound card.) This eliminates unwanted channel noise from other inputs, since they will generate some noise, even if they are not connected or are not being used. This noise will degrade the ARGO display and reduce the ability of ARGO to detect weak signals.
Set the LINE INPUT volume setting to the mid position. Make sure there's nothing plugged into the sound card LINE INPUT jack. Check the ARGO screen. It should be fairly clean, with no vertical lines (indicating the presence of a steady signal) anywhere except below about 500 Hz. A few weak or faint lines above 400 Hz are normal. The presence of many strong lines may indicate interference getting into the input of the sound card from the computer itself.
Try running the Windows Line Input volume slider full up and full down and look at the ARGO screen. Strong signal lines which do not change with the position of the volume slider usually indicate a sound card/computer problem. Try another sound card. If the lines do change in strength as the volume slider is adjusted, the problem is likely interference to the input of the sound card. These lines will probably go away when you have an actual signal connected to the sound card.
Next, set the Windows Line Input Volume slider to about 1/4 volume position. Connect the output of your receiver to the Line Input jack of the sound card. Turn the receiver on, but turn the receivers volume control all the way down. Look at the ARGO screen. If it suddenly has a lot of signal lines visible at power line related frequencies, (USA 60, 120, 180, 240, 300, 260, etc.) you will probably need to install an audio isolation transformer between the receiver and the sound card. You will need to "break the ground loop" caused by the shield of the audio cable connecting the computer ground to the receiver ground. The isolation transformer will do that for you.
Now, turn the receiver on, and tune it to a dead spot on the band so all you can hear is the steady hiss of atmospheric noise (and thunderstorms, and light dimmers, and noisy power lines, and hair dryers, and...) and turn up the receiver volume high enough to get the vertical green volume bar on the left side of the ARGO screen to show about 1/4 high.
If most of the unwanted vertical signal lines go away, then there's no problem; the signal from the receiver is properly overriding the computer noise interference with the sound card. Very likely, you'll see a few other steady signals show up when you do this test. Argo can easily locate signals you can't hear by ear.
At this point, you're pretty much ready to start receiving LOWFER signals.
A few things I have found out about ARGO.
First of all, ARGO "likes" a signal bandwidth of about 400 Hz or more. I have tried using bandwidths as narrow as 20 Hz in an attempt to eliminate unwanted signals, but found that the performance of ARGO degrades somewhat in comparison to using ARGO with a wider bandwidth filter. With ARGO is in the AGC ON mode, the statistical fluctuations of the background noise inside the narrow bandwidth filter window causes ARGO's background intensity level to change fairly substantially, somewhat obscuring weaker signals. If a narrow bandwidth signal is fed to ARGO, the "signal granularity" is excessively large, i.e., the noise does not average out very well. Increasing the signal bandwidth to about 400 Hz gives ARGO enough noise "to chew on" so that it can average out the noise in the background display much better.
It's better to send ARGO a signal that's a bit too weak than one that's too strong. Excessively strong signals can cause distortion in some audio cards, resulting in signal "ghosts" or "echoes" appearing on the display. Blooming of the signal being copied (super "dogbones") can also appear. This will also cause the signal to exhibit apparently excessive sideband energy, spreading the signal on the screen and making it difficulty to interpret the display properly.
I try to have the audio signal sent to ARGO from the receiver adjusted so that the background noise in ARGO indicates the same (or close to) signal strength reading shown on the receiver when the background noise is averaged across a 400 Hz bandwidth. When in doubt, just keep the ARGO signal strength bar somewhere between 1/8 and 1/2 full scale.
If you're going to push ARGO hard, for instance, saving data to a WAV file, fast scroll display, many captures in sequence, etc., you need at least a P150 machine or better.
Here's how to set up the HP3586C Selective Level Meter for use with with ARGO
NOTE: Although the following applies directly to the HP3586, the same concept may be used with just about any other type of SLM or receiver.
Power the unit ON and allow it to perform its normal power-on start-up and self-calibration at 1 MHz.
Press the FREQ button, and enter the desired receive frequency using the front panel keypad. You may enter the frequency in Hz, KHz or MHz.
After entering the desired receive frequency, press the Hz, KHz or MHz button. This sets the selected frequency in the display.
Note that there should be no strong signals present within receivers bandpass (in this case, within +/- 200 Hz of the selected frequency), otherwise the signal will interfere with the calibration process.
Set the remainder of the controls as follows:
AUTO CAL = off
RANGE = 10 dB
FULL SCALE = entry
AVE = on
UNIT = dBm
CHANNEL = >
COUNTER = off
WIDE BAND = off
SELECTIVE =lo noise
TERMINATION = 75 Ohms and set the unmarked button = on
600 OHM = both off
BAND WIDTH = 400 Hz
Set the AUDIO VOLUME knob to the fully CCW position.
Note that there is a set screw visible in the side of the knob at the 7 o'clock position. Rotate the knob CW until the set screw points to the 12 o'clock position.
( This setting seems to give the best signal-to-noise ratio from the 3586 for internally generated noise inside the unit. )
Connect the antenna feedline to the BNC input connector.
Press the FULL SCALE button once.
Press the UP or DOWN arrow buttons until the pointer of the analog signal level meter is indicating between -10 and -9 dB. You should hear a steady hiss from the monitor speaker on the 3586. Note the dB reading in the display window.
This procedure disables the automatic level adjustment feature of the 3586. This prevents a strong signal within the bandpass of the filter from "ducking" a weaker signal if they are both present at the same time. When set in this mode, you will have about a +/- 20 dB operating range around the dB value you have preset.
Press the MEAS CONT button to clear the preset dB reading and display the actual average signal strength reading. This reading is the average of all the signals and noise heard within the 400 Hz bandwidth centered on the preset frequency you previously entered.
Connect the computers sound card LINE INPUT jack to the HEADPHONE jack on the 3586.
Start ARGO running, and set ARGO to MODE = 20s , and SPEED = fast, AGC = on.
Press the START button on the ARGO screen.
Center the displayed frequency at about 1850 Hz.
Note: 1850 Hz is the center of the bandpass window of the 3586, no matter what bandwidth is selected. In other words, if you tune to a steady CW signal at 185.300 KHz, the 3586 will produce a tone of 1850 Hz. 1850 Hz is, therefore, your baseline reference frequency. By using upper side band (CHANNEL >) frequencies ABOVE your selected frequency will produce an audio tone ABOVE 1850 Hz. This tone will be as far above 1850 Hz as the incoming signal is above your selected frequency setpoint shown in the FREQUENCY/ENTRY display window.
What you want to do at this point is to get the averaged dB readings between ARGO and the 3586 to agree within about +/- 6 dB or so. This is not particularly critical, but it does give you the best match between the dynamic ranges of the 3586 and ARGO. To do this, proceed as follows:
Bring up the Windows Volume Control box and turn off all the record inputs except for the LINE IN setting. This eliminates unwanted channel noise from other inputs, since they will generate some noise, even if they are not connected or are not being used. This noise will degrade the ARGO display and reduce the ability of ARGO to detect weak signals.
While watching the green vertical signal level display bar on the left side of the ARGO window, adjust the Volume Control slider on the LINE INPUT control box to obtain a green bar of about 1/6th the length of the bar.
Move the mouse pointer just into the right hand edge of the ARGO window, slightly above 1850 Hz. As the ARGO window scrolls, the mouse pointer will display a red dB reading, corresponding to the signal strength reading of that particular frequency at that time.
Watch the reading for a minute or so, and mentally derive an approximate average reading. If you prefer, you may scroll the mouse pointer up and down close to the right edge of the ARGO blue window. The dB reading will rapidly change as the pointer is moved. You can rapidly make an average "guesstimate" of the dB reading.
Watch the digital dB display on the 3586 for a minute or so and derive a mental average of the reading. Compare it to the reading you derived from the Argo screen.
Using the mouse pointer, adjust the LINE LEVEL input slider in the Windows Volume Control up or down a bit. Go back to the ARGO screen and get another average dB reading.
That's all there is to it. You have successfully set up your SLM and ARGO to work together to pull those weak signals out of the noise. Enjoy!
* I'm an equal opportunity guy. I drive a Chevy, and the XYL drives a Ford. Just to be different, my daughter drives an Olds. We bitch about all of them.
73, Ralph W5JGV
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