Introduction The 2023, 2023A, 2023B, 2024 and 2025 AM/FM Signal Generators cover the following frequency ranges: 9 kHz to 1.2 GHz 2023 and 2023A 9 kHz to 2.05 GHz 2023B 9 kHz to 2.4 GHz 2024 9 kHz to 2.51 GHz 2025 Output levels from -140 dBm to +13 dBm are available. Factory fitted options are available to extend the level to +25 dBm (+19 dBm above 1.2 GHz) and to provide SINAD measurement capability. Fig. 1-1 is a block diagram of the frequency synthesis and signal processing circuits. Synthesizer A VCXO operating at 100 MHz is phase locked to the internal (or external) frequency standard using a phase comparator at 10 MHz. The VCXO signal is divided by 20 to give a 5 MHz reference frequency for the fractional-N loop phase comparator. A fractional-N loop is used to lock a multiplied low-noise VCO to the reference with a resolution of l Hz. The VCO tunes from 400 to 535 MHz and is multiplied by three, four or five to yield a signal in the range 1.2 to 2.51 GHz. A high speed programmable divider is used to divide the multiplied VCO frequency down to 5 MHz and a phase comparator compares this signal with the reference derived from the VCXO. The output from the phase comparator corrects the VCO frequency. In order to provide the required division ratio, the programmable divider is required to act as a fractional divider. The fractional-N gate array controls the division ratio of the programmable divider. The variation of this division ratio by the controller enables the loop to lock, with non-integer division ratios, to the reference with the resolution of 1 Hz without introducing spurious signals. FM is produced using a two-point modulation scheme. The FM signal is inserted into the loop by summing the FM signal with the VCO tune line to modulate the VCO directly. Simultaneously, the FM signal is fed to the fractional-N controller via a 1-bit oversampled A-D converter which converts an analogue input into a bit stream of ‘1’s and ‘0’s. The controller uses this input to modulate the division ratio in sympathy with the modulation. This allows frequencies less than the loop bandwidth, including DC, to modulate the output frequency. In order to maintain good FM performance of the two-point modulation system, the VCO FM tracking characteristics are required to be known. The sensitivity of the FM system via the 1-bit oversampled A-D converter is VCO independent and accurately calibrated by a DC calibration system. The VCO tracking is derived by an automatic FM SELFCAL routine during calibration. During an FM SELFCAL, the error signal on the tune line, for a frequency near the loop bandwidth, is monitored while varying the FM calibration numbers, allowing the variation in VCO sensitivities to be calibrated out. This will remove any perturbation of FM flatness near the loop bandwidth due to mismatch of two modulation paths. RF processing The VCO on board AA1 operates in the range 400 to 535 MHz and feeds to a harmonic generator whose 3rd, 4th and 5th harmonics are selected by voltage-tuned band-pass filters to provide a frequency in the range 1.2 to 2.4 GHz (2.51 GHz on board AA1/1). To generate frequencies below 1.2 GHz this signal is divided by factors of two to produce frequencies in the range 10 MHz to 1.2 GHz. A bank of switched half octave harmonic filters follows which is used to reduce unwanted harmonics at the output. Then the signal passes through the amplitude modulator where the output level envelope is controlled. The output from the modulator is peak detected. For frequencies less than 10 MHz the signal is mixed with an input from the 100 MHz VCXO. The resulting output in the frequency range 9 kHz to 2.4 or 2.51 GHz is fed from the output amplifier to attenuator board AA2. For pulse operation the signal is taken to the pulse modulator which operates in the range 30 MHz to 2.51 GHz. Otherwise the pulse modulator is bypassed and the signal is fed to the output attenuator. This is controlled by relays and provides attenuation in steps of 11 dB up to 132 dB. The attenuator also includes an RPP (Reverse Power Protection) system to protect the instrument from accidental application of reverse power. LF processing The LF processing all takes place on control board AB1 or AB1/1, and may be conveniently subdivided into the following major functional elements. LF output A DSP (Digital Signal Processor) is used to generate the audio frequency signals used for internal modulation. One output from the DSP is used to supply the front panel LF OUTPUT socket. For external modulation the signal applied to the EXT MOD INPUT socket first passes through AC/DC coupling selection and then can either be applied directly or via an ALC (Automatic Level Control) circuit to the audio multiplexers. The external direct signal can also be summed with the audio frequency from the DSP. AM and level control For amplitude modulation the modulation depth is set by a 12-bit A-D converter. A second A-D converter is used to produce the ALC reference for the RF board. Square law correction is applied to both signals. FM/ϕM For frequency modulation the signal amplitude controls the FM deviation. For phase modulation the signal is passed through a differentiator circuit.