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Wednesday, May 29, 2019

Ramsey CT-90 - Portable Frequency Counter

Here is a vintage frequency counter that you can build and use it for learning and experiments in your home or office lab. The front end circuits have to be studied. This will help young enthusiasts learn some basics of instrumentation. - delabs Notes

Ramsey CT-90 - Portable Frequency Counter 

The CT-90 is a laboratory quality frequency counter that is capable of use in the lab as well as in the field. Utmost care has been taken in all the design stages to insure that sensitivity and reliabilty were not compromised. The CT-90 has been specifically built for the critical user, as demonstrated by; special  timebase options, nine digit display, three gate times, and a count hold feature.

Ramsey CT-90 - Portable Frequency Counter Schematic

The CT-90 is ideal for portable usage. Leading zero blanking and the micro power oven (optional) allows 2-4 hours of continuous operation using the internal battery supply.

Operation; Operation of the CT-90 is very simple, simply connect your input signal to the proper input jack (10, 60 mhz or 600 mhz) and select the range and gate time. All switches, except the external time base switch (optional) are mounted on the front panel in easy view of the user. A description of the front panel controls follows.

Construction Notes:Use a small tipped iron for assembly. A power rating of 30-50 watts is ideal. Do not use a soldering gun. Do not use any sort of additional solder flux, use only a good grade of rosin core solder. Proper soldering techniques are important! Each joint should be shiny and completely surround the lead wire. There should not be just a slight dab of solder barely held on to the lead.

Ramsey CT-90 parts list

Regardless of the type or complexity of a frequency counter,all instruments measure frequency by counting input pulses with respect to a known frequency or time base. The time base generates aprecisely controlled time interval, selectable to be one second or one tenth of a second. During this period, the counter is enabled and in¬put pulses counted. When the time period is up, the number of pulses counted is then displayed. A long gate period allows more pulses to be counted, and the more pulses counted the better the resolution.The limiting factors governing resolution are the number of digits in the display and the tolerable gate period. Usually 1.0 hz is the best resolution practical for an easy to read updated count.

Ofcourse it is not always necessary to read frequency to a hertz orwait for a one second count. By selecting a shorter gate period',you can reduce the display update time and get a faster reading display, but at the expense of poorer resolution.

The UHF and VHF inputs have been kept separate to increase the input sensitivity by eliminating switching losses. TheUHF signals are feed thru Jl, past CR1-CR2 input protection diodes to Ql, the first amplifier/limiter stage. The signal is then sent to Ul, divide by ten IC.

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