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Preamp and 330 + MHz Prescaler
for A Little More Serious
Frequency Meter

A preamp that drives the CMOS counter input and a divide by 10 prescaler to extend the range of A Little More Serious Frequency Meter (elsehwhere on www.projects.cappels.org).

(Above) Enclosed in a 16 cm x 16.5 cm plastic box, the preamp has a 60 cm cable to supply power and take the signal to the frequency meter/counter. The cable was originally part of a USB mouse. The labels were printed on a laser printer. Yes, that's clear plastic tape over the labels so my fingers will not rub the toner off the paper.


After finishing Little More Serious Frequency Meter I had planned to make a suitable preamp and prescaler for it, and set about to gather ideas and parts. I was inspired by one fellow who had built the meter and the 2 line X16 character LCD display to show the output, succeeded in designing a preamp based on the BRF96 and modifying the circuit to get it to work at 99.999999 MHz. My intentions are to use the frequency meter between 100 and 200 MHz, so I found a prescaler, the Motorola, now On Semiconductor, MCT10280. For the preamp, I was able to buy some 2N3663 transistors. I would have like to have tried the BRF96, but could not find them stocked at any of my favorite distributors.

What was probably the most difficult part of the design was deciding what I really wanted the circuit to do and how to go about doing it, given the limitations of the available components. The resulting preamp can drive the counter from as little as 20 millivolts P-P input at a few KHz, but needs over 300 millivolts at 20 MHz, and can switch in a divide by 10 prescaler to extend the range to  300 MHz. I have used this at 338 Mhz to date. The data sheet limit for the 74HC4060 is 30 MHz, so performance over 300 MHz is expected by beyond specification.

 A Counter mode allows a direct coupled signal to drive the frequency meter in the counter mode.

The resulting preamp/prescaler design intended to operate within these frequency limitations:

MODE                          DESIGN SPECIFICATION 

Count                         DC to several hundred kHZ (in practice, several MHz)

F/1 Frequency, no prescaler   10 Hz  to  30 MHz 

F/10 Frequency, 10X prescaler 10 MHz to 300 MHz    (Mine worked at 338 MHz.)

These parameters are expected with an approximately 50% square wave up to frequencies of seveal MHz, and symmetric sine waves at higher frequencies. The primary limitation is based on the maximum clocking rate specification for the MM74HC6040 ripple counter in use in the Slightly More Serious Frequency Meter project. Selection of faster parts and careful circuit layout can extend the upper limit of the useful frequency range. The lower frequency imit is the lowest sine wave input frequency for the MCT12080 at which the input of the MCT10290 does not osccilate.


((Above) The cirucit blocks are,from left to right, input protection circuit, prescaler, FET buffer and bipolar limiting amplifier, and Schmitt trigger buffers.

Signals applied to the input connector can be switched either through the AC path which includes the preamp and the prescaler, if switched into the circuit, or the DC path, which routes the signal to a Schmitt trigger buffer that then sends the signal on to the counter.

Regardless of how the signal is routed, it must first pass through an input protection network, which includes two schottky diodes and a zener clamp. The 1N5711 schottky diodes prevent the input signal from going more than a schottky diode drop below ground or above the power supply. I used Schottky diodes because they have a lower voltage drop than the PN protection diodes on the CMOS integrated circuit they are intended to protect, and as such, they will draw much more of the current from excessive input voltages than the input protection diodes in the integrated circuits.

The two 1N5226 zener diodes in series prevents the power supply from rising above 6.6 volts in case the input is accidentally connected to a low impedance source that is higher than 5 volts. The 47 Ohm resistor limits the input current in case of excessive voltage being applied to the inputs.

The input of the frequency meter requires a full 5 volt CMOS logic swing, and the prescaler's output is less than 1 volt peak-to-peak, so the prescaler, when switched into the circuit, the signal goes through the prescaler, then the preamp, and the preamp drives the frequency meter through the Schmidt trigger buffers.

The MCT10280 prescaler can be set to divide by 80, 40, 20, or 10, as a function of which pins are tied to the power supply. I set this one to divide by 10 since it is adequate for my needs, and the mental calculation of multiplying the meter reading by 10 is not taxing.  One problem with the MCT10280 is that if it doesn't have an adequate input, the output is very noisy, which shows up as counts in the couple MHz range on the frequency meter.  This noise shows up if the signal amplitude the signal frequency is too low. For this reason, I only intend to use the prescaler with inputs between 10 MHz and 300 MHz.

Whether the signal is divided by ten or not, it must pass through the preamp. In the Preamp, a 2N5485 N-channel FET is connected as a source follower. This provides a high input impedance to the input signal and drives the next stage, the 2N3663 limiting amplifier with a nice low impedance signal. This results in high AC gain.
DC buffer

Signals from the 2N3663 are nice, clean square waves with fast rise and fall times, and when driving the 74HCT02 input on the frequency meter, could result in the input of the 74HCT02 oscillating. To prevent this, the signal is passed through a 74HCT14 Schmitt trigger. The signal conductor in the cable has an impedance of about 150 ohms, so it is driven through two 300 ohm resistors in parallel to keep the ringing on the frequency meter end of the cable to a tolerable level. There is no termination on the frequency meter end of the cable because terminating it would reduce the amplitude of the signal below the CMOS thresholds.

Power for all the circuit comes through the cable and is regulated by the 78L05 regulator.

Assembly and Test

(Above) The Frequency/Count and F/1 / F/10 switches are mounted on the cover of the plastic box as is the circuit board. The surface mounted MCY10280 along with the 78L05 leaded voltage regulator are mounted on a small subassembly.

The great bulk of the work in assembling this was in cutting the circuit board to shape and making the holes for the input connector, switches, and the cable in the plastic case. After that, it was mostly a matter of placing the components for minimum lead length and hooking them up point-to-point.

The switches could not be mounted on the circuit board because the standoff height was not sufficient and I hadn't anything to extend the standoffs with, so flying leads were used to connect the slide switches to the circuit board. It would have been nice to no have the flying leads, but they appear to not be causing any problems with performance.  I have not investigated the circuit's upper frequency limit, where small differences in layout and wiring could have large effects on performance, except to note that with the prescaler switched into the circuit, the frequency meter is able to measure the frequency of a 100 MHz RF source.

I built up a subassembly with the prescaler and tested it before mounting the subassembly on the main board.  I would have almost been worth making a printed circuit board for this. See the photograph of the surface mount subassembly below.

(Above) The MCT10280 prescaler is mounted along with chip resistors and chip capacitors and the leaded 78L05 on a separate circuit board that was in turn mounted on the larger assembly. The capacitors were taken from discarded cell phones.

(Above)The threshold for square waves is lower than for sine waves at frequencies below 100 Hz because the edges of the square waves make it through the AC coupling. These measurements were taken without the prescaler connected.

Once assembled, I measured the performance and found that I needed nearly a volt of input signal to get the frequency meter to read properly. I added an extra ground wire between the 2N3663 emitter and the ground pin on the input connector, and that helped a little bit. The big improvement came when I shorted the 2.2k resistor on the input of the frequency meter. It is not entirely clear to me why this made such a dramatic difference, but it lowered the threshold to a little over 300 millivolts at 20 MHz.

I used the preamp in the prescaler switch position when I connected it to a buffered Colpitts oscillator I am considering for a future project. As this oscillator was operated from 76 MHz to 306 MHz, the frequency meter read out faithfully in agreement with the oscilloscope, which until I made this probe, was my only way of seeing signals in this frequency range. I have not yet had an opportunity to use the prescaler at higher frequencies.

One note on using it in the prescaler (F/10) postion: The MCT10280 prescaler was designed to be used on a printed circuit board, with ECL amplitude signal sources permanently connected to it. There is ESD protection on the input pins but the details of the EDS protection are not given on the datasheet, so there is no wayto know to what extent the input prtoection network shown in this schematic can protect the MCT10280's input, other than to test several chips to failure. In leiu of any volunteers to perform this testing with their own parts, I can only recomend treating the input carefully when the F/F10 is in the F/10 position. Make sure the frequency meter's ground is connected to the signal source's ground. Be careful not to apply large voltage transients to the input. Know what you are connecting the input to each time.

Clearly, the cable between the preamp and the counter is less than optimum. If the upper frequency limter were to be higher than 20 MHz or the cable longer, a technically better arrangement would be to place a buffer with a driving end termination in the probe body, drive a shielded cable, and add preamp with a receiving end termination to the frequency meter circuit board.

You might also want to see these related projects on www.projects.cappels.org:

A Little More Serious Frequency Meter

Serial Interface for Truly MTC-C162DPLY-2N, 2 line X 16 char LCD display

Frequency Meter and Pulse Generator

100 MHz Modulated RF source

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Contents ©2005 Richard Cappels All Rights Reserved. http://www.projects.cappels.org/ 

First posted in September, 2005. Update 20 Setp. 2005.

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