Protecting a modern spectrum analyser from voltages in valve equipment

 The cost of a spectrum analyser can run into tens of thousands of pounds and modern types often have quite a modest limit on the voltage at the input socket.

My analyser is the DSA815 and it has a limit specified at 50 volts DC and a miniscule 2.25 volts of CW RF and is therefore really unsuitable for use with equipment using valves where the typical HT voltage can be anything from 200 to 350 volts. Of course a transmitter can have even greater voltages present and it's prudent, even if one intends to restrict measurements to points carrying low voltages, to prevent inadvertent destruction of the analyser front end.

It's also worth considering other avenues through which high voltages can get into the input such as connecting to a mains operated radio chassis. Of course it would be foolhardy to experiment with an AC/DC equipment without employing a proper isolation transformer. I say "proper" as there are not only isolation transformers around, but also autotransformers which at first sight appear to be similar to isolation transformers but do quite definitely not provide any isolation. One form of autotransformer might be a device for converting UK mains voltage to US mains voltage. If you use one of these then under certain circumstances you will also need an isolation transformer.

Even if the set being worked on has a proper mains transformer it may have only a 2-wire mains connection. Because of this the chassis will not be earthed and can easily assume a high voltage with respect to the analyser chassis. This can arise from decoupling capacitors connecting a mains input to the chassis and can often be felt as a rubbery sensation through ones fingers. Also present on an old equipment could be an electrical fault such as a bad earthing point.

In summary therefore, connecting a spectrum analyser to a mains powered equipment is fraught with risks if proper earthing is inadequate. See this article

The prudent solution is to always employ a protection device and to this end one of the advantages of building test aids for isolation and voltage protection for a spectrum analyser is the fact you can readily check its performance.

One can of course purchase protection devices, but these are often extremely pricey and, as most are very simple in concept, it is quite easy to make your own. Many amateurs and experimenters are not too concerned with the more esoteric aspects of measurements but mainly interested in relative readings such as determining the shape of a curve. Simple DIY test gear can be built which has really flat characteristics over typical ranges met in receiver or transmitter adjustment, but if you need to work in the realm of precise specifications then commercial test aids are there for purchase.

I'll describe two items which I made very quickly after working out the mechanical details.

The first is a DC blocker and the second an RF probe.

I made both in miniature diecast boxes. The purpose of what follows is to give experimenters a rough idea of what's required and not a article on their detailed construction.

DC Blocker

This is a very simple device which uses two capacitors, a pair of zener diodes and a miniature choke.

 The aim was twofold. First to block a DC voltage getting through and second to limit a spike getting into the spectrum analyser input. I used parts that were readily available so I built it into an old attenuator casing with BNC connectors already fitted. I chose two capacitors so that I could add a clamping circuit that would not interfere with either the circuit under test or the analyser input. I discovered that a pair of 0.22uF 400 volt capacitors worked well in terms of flatness of response. Two 18 volt zener diodes wired back-to-back limit the voltage nicely and to minimise the damping effect of these diodes I added a small choke from a scrap VCR board. The choke is not very critical and can be selected using the spectrum analyser to see its effect. I was concerned about the charging effects of connecting the DC blocker to a high voltage hence the addition of the zener diodes. Connecting these zeners back-to-back clamps both positive and negative spikes to ground and limits the level to around 18 volts which is very much greater than most monitored AC signals.

In the design of the case are four bolts connecting the front and back end plates (removed for the photos).

 

 

High Impedance RF Probe

 The probe's chief purpose is to limit any DC voltage present at the measurement point to something much less than the spectrum analyser's input rating. It's actual amplification or attenuation is to a large extent unimportant as long as the spectrum analyser can cope with the output signal level. I must stress that this type of device is not for the purist, being designed for an amateur experimenter. It will be used mainly for measurements up to 50MHz.

 

The probe is made in a small diecast box and, as you can see, most of the space is taken by the 6 volt pwer supply.

 

 The aim of the design, besides protecting the spectrum analyser from voltages met in valve equipment, was to minimise the effect of the measurement device on the circuit being measured. It's not a good idea to place even a few picofarads across a tuned circuit to observe twiddling effects as, when this is removed, the circuit will no longer be tuned as one desired.

An ideal active device on which to base design is an FET as this will have a very high input impedance (meaning it will draw close to zero power from the circuit under measurement). A surface-mounted FET is best because its geometry will make it even less interactive with the measured circuit. In fact I found I could add a series resistor of 1.1 Megohms without affecting the overall design objective of the probe. This resistance effectively kills off any capacitive effect of the probe, but because, RF'wise, the resulting input circuit is a potentiometer it results in the voltage output of the probe being less than that being measured. Measurements show that the output of the probe with a 100mV input was something like a quarter to a half of this. The spectrum analyser showed that the output of its tracking generator at a level of 0dBm was about -20dBm to -30dBm at the probe output, but this was pretty much flat from 100KHz to 50MHz. Proof of the effect is that connecting the tracking generator directly to the 100pF input capacitor results in a probe output of just marginally less than 0dBM.

 

I drilled a large hole in the end of the box to minimise capacitance. Although a nice material to work with, the diecasting won't solder so I used stand-offs to hold the components in place and a solder tag screwed to the case for ground connections. You can just make out the black case of the FET which has four legs. The wire loop is the connection to gate 2. The drain of the FET is soldered to the stand-off carrying the orange wire which connects to the 6 volt power supply. The pale blue part just visible below the input capacitor is the 10Mohm resistor.

 

 The above picture shows the FET amplifier constructed on four stand off posts. The black capacitor is a DC blocker connected to FET gate 1. The FET is a dual gate BF998 in a 4-pin package for which a brief spec follows:-.

Drain-Source Voltage: 12.0V max.
Drain Current: 30mA max.
Total Power Dissipation: 200mW max.
Forward Transfer Admittance: 24mS typ.
Input Capacitance @ Gate 1: 2.1pF typ..
Reverse Transfer Capacitance (f = 1MHz): 25pF typ.
Noise Figure (f = 800MHz): 1.0dB typ.
Operating Junction Temperature: 150C max.

The basic circuit is a unity voltage gain "cathode follower".The source (cathode) connection has a 47ohm resistor to ground and coupled via a 10nF capacitor and a short length of coax to the rear BNC socket. Gate 1 is biased, via a 10Mohm resistor, by 6.8kohm and 4.7kohm resistors which essentially determine the drain current. Gate 2 (the thin wire link near the orange wire) and the drain are connected to Vcc (6 volts).

The chief practical consideration was how to power the attenuator. The smallest practical diecast box enabled me to fit four alkaline AAA size cells in holders, a small toggle switch and an LED as well as the FET and its circuitry.

The LED hopefully will let me readily see if the probe is on or off. To reduce drain I'm using an 8.2kohm resistor to feed the LED.

 
 Physically building the RF circuit was solved by using stand-off posts. By drawing the various parts on paper I was able to work out drilling details. I assume that anyone wishing to build an attenuator will be able to find suitable components. My only purchases were an FET (plus a spare if something went wrong), a diecast box and a pair of double battery holders.
 

 Using a spectrum analyser with radio equipment designed for WW2 needs some thought. Firstly you need to consider the voltages, which in valve equipment can be around 250 to 350 volts and, because old capacitors can leak it's not good enough to assume you're measuring earthy levels, not to mention finger trouble such as accidentally touching an HT rail with a test probe. Secondly you need to consider the typical RF voltage you'll be looking at. Many oscilloscopes won't display very low signals and 10mV might be their limit, so trying to look at these will be tricky. A decent spectrum analyser should be OK for low level signal analysis but if you have a tracking generator you need to think about its RF level. For example a 0dBm output will be a milliwatt across 50 ohms. This works out at 225mV RMS or nearly quarter of a volt. A communications receiver will have a sensitivity down to a microvolt or less, so pushing quarter of a volt into its aerial socket will be a trifle high. My spectrum analyser tracking generator can be set to a minimum of -20dBm which still represents 22mV. Still a bit on the high side so I add an external 20dB attenuator which results in 2.2mV which is more in keeping with the receiver's sensitivity.

Similarly, when testing an amplifier you should consider its maximum output as it's easy to overdrive an amplifier and produce all sorts of unwanted by-products. Below is a table showing the relationship between common power levels and voltages. Note the red numbers which could be easily met in an RF amplifier.

The figures in red are above the recommended limit for the DSA815.

 Setting in dBm

 Power in 50 ohms

 Volts RMS

 Add 20dB of power

 Add 30dB of power

 Add 40dB of power

 0

 1mW

 225mV

 2.2V

 6.6V

 22V

 -10

 0.1mW

 66mV

 0.7V

 2.2V

 6.6V

 -20

 10uW

 22mV

 225mV

 0.7V

 2.2V

 -30

 1uW

 6.6mV

 66mV

 225mV

 0.7V

 -40

 0.1uW

 2.2mV

 22mV

 66mV

 225mV

 -50

 0.01uW

 0.7mV

 6.6mV

 22mV

 66mV

 -60

 0.001uW

 220uV

 2.2mV

 6.6mV

 22mV

 -70

 0.0001uW

 70uV

 0.7mV

 2.2mV

 6.6mV

 -80

 0.00001uW

 22uV

 220uV

 0.7mV

 2.2mV

 -90

 0.0000001uW

 7uV

 70uV

 220uV

 0.7mV

 -100

 0.00000001uW

 2uV

 22uV

 66uV

 220uV

 

more to follow

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