RF test jig

Designed by sv3ora

I like vintage test gear. The built quality of these old known brands equipment, is no match for todays full-featured, cheaply-built, multi-thousand-Euros equipment. Sometimes, such old equipment, have amazing specifications, even in todays standards, for the price one is called to pay for them. I wanted to use a vintage vacuum tube oscilloscope, sweeper, frequency counter, spectrum analyzer and reference signal generator, to make meaningful measurements in the lab and possibly be able to do more things than my newer limited-budget equipment.

The problem I mainly find with such old equipment, is the lack of markers and the inability of the hobbyist to precisely calibrate them, since he lacks expensive calibration equipment. To overcome both of these problems, the technique i have thought of, is what I call the "comparison method". Instead of having to calibrate precisely each and every equipment, perform just a rough calibration and rely only on a reference calibrated signal generator to do the rest. In other words the "comparison method", transfers the critical calibration to the reference generator only.

The method is quite simple and it is more easily explained with an example. Suppose you want to measure the amplitude and frequency of a test signal, displayed on the screen of an old spectrum analyzer without markers. The way to do it, is to measure major and minor divisions in the display graticule, then take into account the amplitude and frequency settings of the spectrum analyzer. Depended on how old and calibrated the analyzer is, you can get from adequate results to totally wrong, but never precise. Apart from analyzer errors, setup errors or losses will also apply to an extent.

With the method of comparison, nothing of the above matters. The method, alternatively compares the test signal on the analyzer display, with the reference generator signal on the analyzer display, alternatively applied to the input of the spectrum analyzer. By closely matching the amplitude and frequency of both of these signals, the amplitude and the frequency of the test signal is read on the reference generator screen. It's as simple as this. The same teqhnique applies also in the time domain (oscilloscope) to measure amplitude and frequency, although more tricky for frequency, taking into account the distortion of the test signal. However, we do not need to worry for the relative phase difference of the two signals, because the scope trigger will take care of this.

The first schematic in the picture below, shows a circuit that I have built to exploit te comparison method. It is a simple passive two-switch device that allows instant switching any of the ports on the left, to any of the ports on the right. For example, you can connect the test signal and the reference generator to the left ports and the spectrum analyzer and a frequency counter to the right ports. By single switches flip, you can instantly select either the test signal or the generator to be sisplayed on the analyzer display. If your reference generator has no frequency display, you can select the counter instantly and measure the frequency of the generator, after you have matched it to the test signal.

The device can be used bidirectionally and in many ways to achieve the desired test setup. Depended on the operating frequencies (my interest was on HF) you might want to build variations of the circuit with coaxial relays. Note however, that any losses will apply to both the test signal and the reference signal, so effectively they cancel out. I would say that the important thing, is the leakage between the switches, rather than their operating frequency or loss. We have to be very careful what we compare, if we are to get accurate results. Thus, another important thing, is that the pair of coaxial cables that connect the equipment on the left, must be of exactly equal length and type. Similarly,
the pair of coaxial cables that connect the equipment on the right, must be of exactly equal length and type. But the pair of cables on the right, do not need to be of equal length and type with the pair of the cables on the left. The device will not only provide you with measurements and connections flexibility, but it will also save your precious connectors from wear by continuously plugging them in and and out.

Apart from the markers/measurement ability, I wanted a test jig that could be used to accurately measure the response curves of devices such as filters, amplifier blocks, cables etc. Therefore, I designed the circuit at the bottom of the above image. This passive circuit is a very useful addition to any lab, that has a calibrated amplitude sweeper with ramp output and a low bandwidth oscilloscope. Old timers, TV and radio technicians, used signal tracers and generators/sweepers and detector probes to properly peak the IF filters in these devices. My circuit differs a bit in the sense that it can be used not only to peak or see the response, but also to actually measure it.

You dont need a spectrum analyzer to perform these frequency domain measurements, your old low bandwidth oscilloscope will suffice! The RF output of the calibrated amplitude sweeper, is applied on the left port. The device under test (DUT) is connected to the DUT port. Do not forget to connect the ground to the DUT too. In my version I used aligator clips with short leads for these connections (interested on HF frequencies). S3 switches the DUT inline or bypasses it. On the right side, there is the peak detector (demodulator) that converts RF to DC. I used the 1N58A Germanium diode for its low voltage drop and huge reverse voltage breakdown (100-120v). The detector is a sligh variation of an old General Electric circuit. The oscilloscope is connected to the right port and make sure that the coupling of the oscilloscope is set to DC.

The potentiometers are a very important element of the circuit. They are used as input and output impedance set for the DUT. This is very important. In the picture below, you can see the response of a 455KHz mechanical filter when not properly terminated in its input and output. The filter is supposed by design to have a relatively flat response, and this is by far non-flat.

n the picture below, you can see the response of the same filter, when the input and output potentiometers are set to more proper values. This is quite a difference, isnt it?

A side effect of the potentiometers, is that they vary the amount of signal applied to the DUT and also to the detector. A better approach would be to use other impedance matching elements such as inductors and capacitors, or active elements. However, a variable resistor is easily set up, it is more frequency independent and pretty much of known impedance values, since the inductive and capacitive component is low.

To better understand how this jig works, I will give you an example of measuring the response of a filter. The sweeper is set to manual sweep. That is, the user moves the frequency knob manually which results in manual control of the sweep ramp. The sweep ramp signal is connected to the horizontal input of the oscilloscope, or to the horizontal control of an XY display. The oscilloscope must be of storage type for manual sweeps and for this method to work, like the Tek 564 for example. S3 is set to put the DUT inline. Then the user manually sweeps across the filter frequency and the filter response is shown on the CRT of the oscilloscope.

The next step is to trim the input and output potentiometers to achieve the desirable filter response. Several manual sweeps may be required during this phase. When the response is the desirable, erase the CRT and make a final sweep. This is the response curve of your filter. To measure the frequency at any point onto the curve of the filter, lower down the intensity of the oscilloscope beam to the point where the dot is just visible, but not be able to "write" to the screen. Then sweep across and bring the dot to the desirable curve point. Measure the frequency of the sweeper at that point with a frequency counter, if your sweeper does not have a digital frequency display readout.

Next, I like to draw some reference lines on the storage CRT, to more easily measure the amplitude-related things of the filter. To do so, increase the beam intensity, switch S3 to bypass the DUT and without changing the values of the potentiometers (important) or the RF level of the sweeper, perform another sweep. This sweep line is your top reference line and it should be equal or at higher position than the top curve of the filter. Reduce the RF amplitude of the sweeper by the step-size you want (eg 1dB, 3dB, 6dB etc) and draw another line. Then another line by less level and so on.

The picture below, shows these horizontal level lines. Each one is at 1dB difference compared to the nearby ones. Notice that the spacing of the lines is not linear. This can be partially due to oscilloscope mis-calibration and partially due to detector non-linear response on different signal levels. However, it does not matter much, since both the curve and the lines are drawn with the same conditions.
It does not matter if the detector is linear because any non-linearities will be shown on these horizontal lines. So if you see a slight notch on the curves and a slight notch on your filter response, this meas that the notch is from the detector and not from the filter. This might be noticed in wideband filters. In low bandwidth IF filters any detector non-linearities are not present.

The top reference line level is unknown and it is not the same as the sweeper output level, since these potentiometers vary this level anyway. However, we know that the line below it, is at -1dB from that reference level and the next lines -1dB each. Therefore we can measure the filter bandpass loss in dB, as well as the overal response of the filter. We can also fine-measure any amplitude within these steps, by bringing the beam intensity down so that the beam is barely visible and not able to write on the CRT. Then manually varying the calibrated amplitude of the sweeper between these amplitude steps and also perhaps the frequency of the sweeper, to bring the dot onto the curve if desired.

Once the DUT has been measured and when you are happy with its curve, based on the I/O impedances, then remove it from the aligator clips and make sure S3 is flipped to the DUT position. Then measure the resistance from the clip marked "to DUT" to the sweeper port (with the sweeper disconnected of course). This is the series input impedance that you must include in the circuit your DUT is to be installed in, so as to achieve the same response. Then
measure the resistance from the clip marked "from DUT" to the ground. This is the shunt output impedance that you must include in the circuit your DUT is to be installed in, so as to achieve the same response.

If your oscilloscope is of low bandwidth, you can still do power measurements if you combine the top and the bottom circuits. You can connect your reference generator to one port on the left and the test signal to the other on the left. Then connect one of the output ports to the input port of the detector and bypass the DUT. You will then see two lines on the oscilloscope CRT as you switch between the test signal and the generator one. Set your generator amplitude to match the two lines on the screen. Then read the amplitude value on your generator.

The first picture on this page, shows the way the jig was built. Both circuits were placed inside the same die cast enclosure. All the components used, are of the finest quality. Silver plated connectors, mil-spec toggle switches, sealed potentiometers and knobs. The aligator clips cables are silicone-insulated, but I could not find good quality aligator clips.

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