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RF-T-PrescalerMC12093.php    8207 Bytes    01-09-2020 06:33:54

Prescaler with the MC12093   :2, :4, :8 up to 1 GHz


The assembled prototype

✈ Motivation

This circuit is that universal, that you always wanted to have one. Even so we integrated it in many designs, a limited meaurement range of a Microsemi Phase Noise Test Bench triggered the design of a standalone version.

This standalone prescaler is not very power-hungry (≥ 8 V, 10 mA) and the divider value may be changed by assembly / disassembly of two different resistors. (:2, :4, :8, see schematics or datasheet)

You may use it to extend the measurement range of many devices, e.g. frequency counters or to create harmonics and filter out the interesting one.

✈ Downloads

✈ Tendency to Oscillate

The Circuit has an input amplifier with a very high gain, as it has to deal with signals from -10 dBm up to +10 dBm. Therefore it mayst happen, that the circuit starts to oscillate, when nothing is connected at the input. If your setup always has a signal at the input, proceed to next paragraph.

Original (Top level : 0 dBm)
Calmed Circuit (Top level : -30 Bm)

We used a Resistor of 470 kΩ at the unused input to unbalance the working point of the input amplifier. After the resistor has been added, we measured a DC voltage of 3.40 V and 3.43 V. Tendency to oscillate was reduced drammatically by that. You may want to reduce the value of the resistor further, at the price of a reduced sensitivity.

✈ Applications

As with other "handy" things, you may use them for different applications.

It is a well known secret, that the phase noise of an oscillator improves by 3 dB with every division by two. That's how you build a synthesiser powerful enough to challenge (and beat) the venerable hp 8640.

Another fact is, that a square wave (which is produced by the digital divider circuit inside) is rich in harmonics. Therefore one could synthesise a frequency by dividing a good quality crystal oscillator and filter out the desired harmonic.

Input 1 GHz, Output n * 250 MHz
(Detail) -42 dBm at 2 GHz

Another application is the extension of the Measurement Range of a Phase Noise Test Setup or Frequeny Counter.


Picture from D goes here :-)

✈ Interfacing with TTL

The output of the Prescaler is slightly to low to be directly connected to TTL circuits. A buffer with some gain is necessary. The datasheet specifies a typical output voltage of 800 mVpp up to 350 MHz, decreasing to 450 mVpp at 550 MHz.

We usually use a Buffer made with a BFR92A (Rc = 360 Ω, Rcb = 3.6 kΩ), capacitively coupled at the input, dc coupled at the output. Supply is 5 V.

Output at 25 MHz (100 MHz : 4), as seen on a Scope

✈ Share your thoughts

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t1 = 6586 d

t2 = 188 ms

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