MODEL 8559A

SERVICE

Regardless of which harmonic is used for mixing, image frequencies can create problems. Image frequencies occur when a signal not in the band being viewed mixes with the LO to produce a response. It is possible to be in the 1 - band and have a signal at 5 GHz produce a response at 2 GHz; the opposite can occur in the 1 + band. As can be seen, it is necessary to be able to differentiate these signals. In the H P 8559A, this is the function of the signal identifier.

Signal Identifier. Several methods of eliminating image responses are used in spectrum analyzers: low-pass filters, preselectors, and signal identifiers. Low-pass filters eliminate all upper out-of-band frequencies from the mixer; this works well for single band analyzers. A preselector (a YIG-tuned bandpass filter) tracks the LO frequency; this allows multi-band operation, but can degrade input sensitivity. The signal identifier allows identification of in-band signals without losses in sensitivity. This is the scheme used in the H P 8559A.

Signal identification simultaneously shifts the display frequency down 1 MHz and decreases the display amplitude about 5 dB. If the signal is an image, it will do something other than shift down 1 MHz. The SIG IDENT button on the front panel activates this function by simultaneously shifting the frequency of the second LO and varying the level of the video signal during alternate sweeps.

IF Section

The IF section comprises the third IF filters and amplifiers, and the step gain and logarithmic amplifiers. It also includes the video detector, video filters, and video amplifiers. The IF section processes the 21.4 MHz output of the Third Converter Assembly A10 and applies it to the vertical deflection circuitry in the display mainframe.

The 21.4 MHz third converter output is processed by the Bandwidth Filter No. 1 Assembly A1 1, the Step Gain Assembly A12, the Bandwidth Filter No. 2 Assembly A13, and, finally, the Log Amplifier Assembly A14. Each assembly occupies a separate printed circuit board, which is shielded by extrusions mounted on the Mother- board Assembly A16.

Bandwidth Filters. Bandwidth Filter No. 1 Assembly A1 1 and Bandwidth Filter No. 2 Assembly A13 are identical; each contains two synchronously-tuned filter poles isolated by buffer amplifiers. Synchronously- tuned filter poles have identical center frequencies, unlike stagger-tuned poles. The bandwidth of these poles, varying from 3 MHz to 1 kHz, is changed simultaneously by the front panel RESOLUTION BW control. Because the variable bandwidths are so much narrower than any of the RF section bandpass filters, the RESOLUTION BW control setting determines the analyzer's overall bandwidth. Parallel LC filters provide bandwidths from 3 MHz to 100 kHz. Crystal filters provide the narrow, 30 kHz to 1 kHz, bandwidths.

Step Gain Amplifier. Located between the bandwidth filter assemblies, the Step Gain Assembly A12 provides precise and selectable gain in three stages, a 10 dB stage followed by two 20 dB stages. Each stage can be turned "on" for full gain or "off" for unity gain. By turning on the amplifiers in combination, gains of 0 to 50 dB may be selected. This action is performed by the REFERENCE LEVEL control. Concentric with the REFERENCE LEVEL knob is the REF LEVEL FINE potentiometer, which controls the 0 to 12 dB PIN diode attenuator. In addition to the gain circuits described, circuitry providing biasing to the first mixer diode and flatness compensation to the third converter is included on the Step Gain Assembly A12.

Logarithmic Amplifier. The second bandwidth filter is followed by the Log Amplifier Assembly A14. The gain of this amplifier is a logarithmic function of the input signal, which allows a greater range of signal amp!itudes to be simultaneously displayed on the CRT. This logarithmic amplification of the signal before detection results in the vertical display axis being calibrated in decibels (relative to a milliwatt), rather than volts. Linear amplification from 0 dB to 40 dB may also be selected from the front panel.

The video detector, located on the Log Amplifier Assembly A14, is basically a half-wave rectifier and a filter. This circuit produces a voltage proportional to the signal level, called the video signal. This signal passes through a video filter and a vertical deflection amplifier before leaving the H P 8559A.