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

This higher harmonic mixing mechanism is one of the main causes of residual responses. Since the possible harmonic numbers are up to infinity, isolation between the first mixer and the second mixer is very important even at unused frequency ranges. Therefore, it is desirable that the mixer and the second mixer be located on separate blocks. It is also necessary to insert a low-pass filter between the two mixers. Most spectrum analyzers have an independent low-pass filter between the first mixer block and the second mixer block.

The HP 4396A includes all of the filters needed to reject all three of the undesired effects mentioned above on one board. A well-designed, multistage, low-pass filter and a low-cost, high-efficiency RF shielding method made this possible. The resulting residual response in the HP 4396A is -100 dBm for customers specifications and less than -110 dBm at production. The spurious response specification is -70 dBc.

IF Detection

The 21.42 MHz from the second If filter is converted to lower frequencies so it can be handled by the 80-kHz, 16-bit ADC. Depending on the instrument settings, there are three modes for IF detection:

* Dc sampling mode, which is used in the spectrum measurement mode for wider resolution bandwidths (RBWs)

* FFT (fast Fourier transform) mode, which is used in the spectrum measurement mode for narrower RBWs

* Ac sampling mode, which is used in the network measurement mode.

DC Sampling Mode. The components involved in providing the dc sampling mode in the HP 4396A are shown in Fig. 7.n This mode is used during spectrum analysis with resolution bandwidths in the range 10 kHz [less than or equal to] RBW [less than or equal to] 3 MHz. In this mode, the signal from the second IF filter is sent to the two mixers shown in Fig. 7. The LO frequency for this stage is the same 21.42 MHz passing through the second IF filter. The two local oscillators have in-phase and quadrature-phase relationships so that the two IFs in this section are the in-phase and quadrature components of the second IF.

The in-phase and quadrature components are sampled simultaneously by the two sample-and-hold circuits. The sum of the square of these two components is calculated by the digital signal processing (DSP) chip and then transferred to the CPU. The CPU detects the data as the input power. RBWs of 1 MHz and 3 MHz are shaped in the second IF stage (21.42 MHz) by bandpass filters, and RBWs of 10 khz, 30 kHz, 100 kHz, and 300 kHz are shaped by the switchable low-passd filters in the third IF section.

FFT Mode. THE Values and behavior of components in the FFT mode are shown in Fig. 8. This mode is used whmn the RBW is set between 1 Hz and 3 kHz. The FFT mode provides two great advantages. One is that the digital FFT filter replaces the analog filters for narrower RBWs. The analog filters h would have required a very careful design and many precise adjustments resulting in a higher production cost. The other advantage of the digital FFT filter is the sweep speed. The sweep speed is very fast because the FFT algorithm can get information over a 10-kHz bandwidth at one time with the desired resolution, while conventional IF detection provides information at only one frequency at a time. The FFT method is desired in more detail on page 90.