Receiver design for a combined RF network and spectrum analyzer - radio frequency; HP 4396A - includes related article on digital signal processing techniques - Technical

Hewlett-Packard Journal, Oct, 1993 by Yoshiyuki Yanagimoto

It was also found that gluing two pieces of microwave absorber on T4 and T5 on both sides of the board reduces distortion and LO feedthrough. This is probably because the LO signal coming into T4 and T5 through air; which would cause an unbalance of the driving signal, is absorbed by the absorber material.

These efforts reduced second-order harmonic distortion to less than--70 dBc at -30-dBm input level and LO feedthrough to less than -15 dB from the maximum input equivalent level.

First IF Amplifier Design

The first If amplifier is a GaAs FET amplifier with the appropriate impedance matching. It has a 17-dB gain centered at 2.06 GHz.

The IF amplifier was designed using the HP Microwave Design System (MDs), which is a very powerful software tool for microwave design and simulation. Although the designers did not have a great deal of experience with the MDS, the IF amplifier was designed very quickly without any trouble. The short design time was accomplished with the prototyping system, which links the MDS data file with printed circuit board fabrication. This system allowed one-day delivery of the prototype board, or in orther words, the simulated circuit became a printed circuit board the next day.

First IF Filter Design

IF filters are used for three main reasons. One is to reject the image frequency of the second IF. Usually, a bandpass filter is used for this purpose. Another is to remove all the higher harmonic products that are generated in the first mixer. A low-pass filter is used for this purpose. The third reason is to prevent LO feedthrough and unwanted products of the first mixer from distorting the IF amplifier. This is solved by locating the bandpass filter mentioned above between the first mixer and the IF amplifier. The first If block in the HP 4396A is shown in Fig. 5.

The image-rejecting bandpass filter is essential in a spectrum analyzer. For example, the 2.05858-GHz first IF must be converted down to the second IF, 21.42 MHz, with the second LO at 2.08 GHz. The image frequency in this case is calculated to be 2.08 GHz 21.42 MHz = 2.10142 GHz. If the image rejecting filter were not in place, two signals would appear on the display 42.84 MHz away from each other. One would be real and the other would be an image. The 2.10142 GHz should be removed completely by the IF bandpass filter with 2.05858 GHz being passed through. The requirements for the IF filter are very strict and had it not been for the high-Q dielectric filters, the IF filter stage could not have been built. Actually, two dielectric filters in series are used to reject the image. A major advantage of this design is that one conversion stage was omitted that would otherwise be necessary. This contributed greatly to lowering the cost of the receiver design.

A harmonic of the first LO (2.05858 GNz to 3.85858 GHz) and a harmonic of the second LO (2.08 GHz fixex) can mix together to create a signal equal to the frequency of the second IF (21.42 MHz). This will cause a residual response. Fig. 6 shows an example of a residual response caused by mixing the fifth and seventh harmonics from two oscilators. If the instrument is tuned to 857.704 MHz, the first LO is set to 2916.284 MHz. The fifth harmonic of this is 1458.42 MHz. The seventh harmonic of the second LO is 14560 MHz. The difference of these two harmonics becomes 21.42 MHz, which would be detected as a fake signal. The fifty-seventh harmonic is just one of many harmonic combinations.