Home built universal Frequency Counter

In 2005 this counter was for sale on a Dutch internet auction site. I liked the looks of it and made a bid. It turned out it has been built by a German radio ham, who has left his call sign on the front. The chassis is neatly made of sheet aluminium, the constructor has added a pair of handles to the front panel to make it look cool.

The counter can measure frequency and time and can handle up to 250 MHz. Two knobs on the front are used to set gate time and select one of 6 functions:

• Test function (have to find out what this is),
• Frequency of the input signal,
• Period of the input signal,
• Time (using separate start and stop signals),
• Event counter,
• Digital Clock, using only 6 digits, lighting two decimal points and making the counter count in 60's and 24 instead of 10's.

Gate times can be set from 0,1 μs to 10 s. The counter has two inputs:

• A 30 MHz input
• A 20-250 MHz input with a high-speed divider.

Three push buttons on the front are used to reset, start and stop the timer or clock. Two extra TTL level inputs can be connected to external start and stop signals.

Most of the circuitry is mounted on a single large circuit board. This board has been made by the constructor of the counter, as it is bearing his call-sign. Looking at the quality of the board (it has tin-plated traces), I get deep respect for his skills. Electroplating a PCB, probably using the tin coating to protect the copper not to be etched away is a sophisticated way of PCB manufacturing.

This counter has a lovely nixie tube display consisting of:

• 7 Valvo ZM1000 nixies for numeric readout.
• 1 Philips ZM1002 nixie to display units

The logic circuits are using TTL circuits:

• SN74141 nixie decoder/drivers,
• SN7475 quad latches to hold the display while the counters are counting,
• SN7490 asynchronous decimal counters,
• SN74196 synchronous decimal counter in the first counter stage,
• and more TTL IC's to form the control circuits and oscillator.

The production dates on the IC's and power transistor are from 1973 and 1974 dating this counter in 1974 or slightly later.

The control circuit consists of 2 SN74121 monoflops, a 74S00, a 7400, a 7451 and a 7413. The counter has a 1 MHz crystal as frequency reference and a series of 7 SN7490 dividers to produce the different port times and timing frequencies.

For the 250 MHz input, a fast counter chip is used as a pre-scaler. This is probably an ECL chip, as it has a heat sink on top. It is placed on a separate pre-scaler board situated right behind the 250 MHz input below the main board.

Under the main board there is an additional PCB that holds 5 extra 7400 chips as reset circuitry for the clock function.

The power supply is using a small (25 VA) transformer, that looks like it has been custom wound. The label states it can deliver 8.5 V at 1.5A and 170 V at 50 mA. The constructor has added an extra 15 turns of wire to boost the 8.5 V secondary. The +5V is regulated by an LM305 buffered by a 2N3055 and 2N2807. The power transistor is running quite hot as does the transformer. I think the power supply may be a bit underdimensioned.

After I checked the state of the components, I switched on the counter. It just worked and showed a series of pretty zeroes. When I tried the Clock mode, that just worked. Setting the time requires some fiddling with the period selector and the start/stop buttons. I will have to recallibrate the time base and see if I can find a purpose for this neatly built DIY counter that deserves to be kept in honour.