I finally got in my BNC connectors and 2P6T rotary switches and have the circuit board for the HP-531xx frequency counter calibrator just about finished. This board lets you fully calibrate the HP-531xx counters including the "fine" time interval calibration and the channel gains. The basic circuit mimics the (rather unobtainium) HP J06-59992 time interval calibrator. It takes a 10 MHz input, feeds it through a sine-to square converter (using a biased CMOS gate) doubles it to 20 MHz using XOR gates, divides it by two using a D flio flop (the curcuit also supports an on-board canned 20 MHz oscillator). The board is powered by 9-12 VDC via a 2.1 mm barrel connector... it should be able to run from a 9V battery. The 10 MHz Q and /Q flip flop outputs are fed to pairs of paralleled 74AC244 buffers and reduced to +/- 0.5V signals using a series cap and two resistors. These 4 outputs are the P1, P2, N1, and N2 signals generated by the 59992 A/B/C/D switches. These signals (along with an open circuit for the OFFSET calibration and a 5.000V reference voltage from an LM4040 for the GAIN calibration) are fed to the counter CHA and CHB inputs via a 2P6T rotary switch. The CHA and CHB BNC's are spaced to match the connectors on the 53132A so you can connect it to the counter with either male-male BNC adapters or cables.