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

A low noise, floor, reduction of distortion and local oscillator feedthrough, and removal of image frequencies and higher-order harmonic products were the main design objectives for the HP 4396A receiver.

To keep receiver cost low, the HP 4396A 1.8-GHz vector network and spectrum analyzer uses only one receiver for both netwok and spectrum measurement modes. This article describes the design and implementation of some of the main components of the HP 4396A receiver.

A block diagram of the receiver portion of the HP 4396A is shown in Fig. 1. The receiver is located on the HP 4396A's RF converter board and IF board. A signal enters the RF converter board and hence the receiver via the GaAs FET switch either through the S (spectrum) input after the step attenuator or through the R, A, or B ports located at the input

to the input multiplexer. The selected input signal enters the first mixer through the input low-pass filter. The signals is converted to the first IF frequency (2.05858 GHz) in the first mixer by the local oscillator (LO) whose frequency range is between 2.05858 GHz and 3.85858 GHz (tuned to correspond to RF input frequencies between dc and 1.8 GHz). A dielectric bandpass filter with a dummy load is connected to the first mixer to reject LO feedthrough and the unwanted products that might distort the first IF amplifier.

A 17-dB gain is provided by the first IF amplifier. The noise performance of the receiver is mostly decided at this point. The total sum of the conversion loss and the insertion loss of the circuits before the IF amplifier is between 13 and 17 dB. The input noise of the IF amplifier gives a system noise floor of about -155 dBm/Hz to -151 dBm/Hz.

After going through a cascaded low-pass filter that rejects the higher harmonic products, the amplified signal is amplified again by an identical amplifier and then converted to the second IF frequency (21.42 MHz) by the second 2.08-GHz local oscillator in the second mixer.

The second IF is converted to either dc or 20 kHz, depending on the measurement mode, and then converted to a digital signal by the 16-bit, 80-kHz analog-to-digital converter (ADC). The digital signal is transferred to the digital signal processor (DSP) chip. The DSP calculates vector ratio, FFT, logarithm, average (digital filtering), and display decimation. The video filters are also implemented in the DSP chip.

First Mixer

The characteristics of the first mixer in the HP 4396A set the standard for almost all the distortion and noise performance of the instrument's network and spectrum measurements. For this reason a lot of time and effort was put into the design of the first mixer.

The input noise floor of the receiver is mostly determined by the first mixer. The conversion loss is the most significant factor affecting the input noise. Distortion is another very important factor. Harmonic distortion is generated in the first mixer, while third-order intermodulation distortion (IMD) is generated in all of the stages of the receiver.

Gain compression is also caused by the nonlinearity of the mixer and the succeeding circuits. For a network analyzer, compression is a more frequently used concept than distortion, which is more commonly used in relation to a spectrum analyzer.

Dynamic range is defined as the ratio of the maximum input level to the equivalent input noise floor. The lower the input level, the smaller the distortion and the compression become. The maximum input level is defined so that the distortion or the compression is within the instrument's specifications. To meet compression specifications, the maximum input level for the HP 4396A's first mixer is -10 dBm. For two-tone input signals the maximum input levels is -30 dBm to meet the third-order IMD specification of -80 dBc.

Fig. 2 shows the conversion loss of the first mixer and the input low-pass filter. The first mixer and other circuits, such as the input attenuator, the input switch, the low-pass filters, and the cables, make the total loss (sum of the insertion loss and the conversion loss) about 9 dB to 13 dB. The dielectric filter right after the mixer gives another 4-dB loss before the signal is amplified by the first IF amplifier. The noise figure of the first IF amplifier and the overall loss that accumulates before the amplifier sets the system noise floor for the HP 4396A at -155 dBm/Hz to -151 dBm/Hz.

Two approaches can be used to make the dynamic range wider. One is to increase the maximum input level. The other is to reduce the conversion loss, that is, lower the equivalent input noise. The approach chosen for the first mixer in the HP 4396A was to reduce the conversion loss.

It is well known that a double-balanced mixer has less conversion loss than a single-balanced mixer. However, HP had never made a double-balanced mixer in this frequency range. This is because the complexity of a double-balanced mixer makes the second-order harmonic distortion and the

LO feedthrough worse than a single- balanced mixer, which has a much simplier configuration.