Dr.Ing.Jochen Jirmann, DB1NV

A High-Precision Logarithmic IF Amplifier

VHF Communications 3/1996

A logarithmic intermediate-frequency amplifier was proposed years ago for the spectrum analyser for 10.7 MHz, with a dynamic range of approximately 70 dB. The circuit’s cost was considerable. A better result can now be achieved, for a lower cost, using an integrated module from Analog Devices.

1. INTRODUCTION

About 7 years ago, when the author’s spectrum analyser project was entering its final construction phase, it was worth finding the simplest possible logarithmic intermediate-frequency amplifier. The limiting conditions were: 70dB dynamic range, band width at least 200 kHz. The existing solutions at that time were the expensive logarithmic intermediate-frequency amplifier from DL8XZ and a log amplifier using a Plessey integrated circuit, SL521 or SL523. The latter variant can be found in relatively simple radar equipment for marine navigation, and requires four SL523’s. The intermediate-frequency amplifier from DL8XZ was eliminated, in spite of the high precision of measurement and the dynamics, due to the space required and the cost of smoothing.

The author next worked out a variant of the DL8XZ amplifier with differential stages and high-frequency side summation, which was presented in issue 2/1987. It was certainly almost free of smoothing, but the cost of 18 transistors for 70dB dynamics was considerable.

In the second version, therefore, the author used an FM intermediate-frequency circuit with a logarithmic field strength measurement output. The choice fell on the TDA1576 from Philips.

Very little expense is involved and the logarithmic precision is fully sufficient for radio equipment (owners of Japanese radio equipment can only dream of such S-meter precision), but the short waves still stand out in an unpleasant way on a gauge.

2. NEW MODULE

Anyone who would now like to improve the precision of level measurement of his or her spectrum analyser, or who is planning to build a measurement receiver, can now switch to an integrated logarithmic amplifier from Analog Devices, the AD606. The module is available in a DIL-16 housing or in an SO16 surface-mounted housing. According to information from Sasco, the individual unit costs approximately DM 115 plus VAT.

Fig.1 shows some brief data and a block diagram

Apart from the 80dB dynamics logarithmic amplifier, the AD606 also contains a power-down control using a logic signal (not of any interest here), an integrated low-pass filter, the limiting frequency of which can be adjusted using external capacitors, and an output for the restricted intermediate-frequency signal. The data sheet gives the logarithmic precision at ±1.5dB max. over the entire level range from -75dBn to +5dBm, with a typical error of ±0.4dB.

As it happens, the output voltage range from 0.5 to 3.5V is perfectly suited to the output level of the TDA1576.

3. SPECIMEN CIRCUIT

As shown in Fig.2, the cost of the external wiring is extremely low. Apart from a few blocking capacitors, only a 5V voltage regulator is used. Anyone who invests in two resistances and two trimming potentiometers can make both the offset and the log characteristic line gradient adjustable.

The small number of components can be assembled on one experimental printed circuit board measuring approximately 30 mm x 40 mm. The assembly can then be fitted, through the TDA1576, into the high-frequency / intermediate-frequency section of the spectrum analyser.

Alternatively, the TDA1576 can also be unsoldered, and the AD606 can be directly hand-wired in, when the old tracks have been removed. In no case may 12 to 15V be fed directly to the AD606. The maximum supply voltage for it is 9V. The input signal for the AD606 is measured off at pin-15 of the TDA1576.

4. MEASUREMENT ASSEMBLY

To compare the two logarithmic amplifiers, the spectrum analyser was set to 100 MHz and 200 kHz band width, the high-frequency signal being generated by an HP8640A with a downstream precision reference circuit (DPU from R & S). Fig.3 shows the assembly.

A switch installed as an aid makes it possible to connect both video outputs to the display section of the analyser. To plot the characteristics, the analyser worked in measurement receiver mode (zero span), the output voltage from both log amplifiers being measured by a digital voltmeter (Fluke 8800A).

The improvement in the precision of measurement could be recognised simply by looking at the flanks of the spectrum line: for the AD606, the flanks are smooth, and the slight waves well- known from the TDA1576 are completely absent. A total dynamic range of at least 70 dB can be used with a filter band width of 200 kHz.

Plotting the logarithmic characteristics in Fig.4 confirmed the visual impression. The characteristic of the AD606 is almost a straight line, and does not have the inherent waviness and the non-linearity at high levels of the TDA1576.

5. SUMMARY

Anyone wishing to improve the precision with which levels are measured by the spectrum analyser from DB1NV can obtain almost professional accuracy of measurement by using the AD606. The maximum expenditure involved here is DM 150 - plus a few hours’ work.

The AD606 is also interesting with regard to the modernisation of elderly spectrum analysers or measurement receivers, since it is suitable for intermediate frequencies from a few kilohertz up to 50 MHz. It could itself be used as a substitute for the problematic hybrid log amplifier in the 8755 HP scalar analyser, because it can process the 27 kHz intermediate frequency directly.