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DIY Mini VHF Frequency Display (up to 400 MHz)

Yet another frequency display. Yes. As it seems that there are mainly pic-based designs out here, we hacked together a version based on the Atmega 8 (Atmel). A prescaler (MC12080) does the downscaling into a 'counteable' frequency range.

DIY Mini VHF Frequency Display

Photo shooting with the prototype. And yes, it's a mirror.

The Concept - Functional Description

The concept here is : keep it simple. Therefore only two IC's, a buffer and a LCD are used. All magic happens on a single sided pcb. (One bridge was necessary).

The input signal is ac-coupled to one port (Pin8) of the MC12080 prescaler. Two of the three switches are connected to the Atmega. Therefore the divider may be switched between 20, 40 and 80. This is done depending on the input frequency (firmware upgrade :-). The level of the RF may be in the range of -10 dBm to +10 dBm.

Following is a buffer, made with a BFR92 high frequency transitor.

The signal is then fed into the Atmega at Pin 11, PD5, T1.



As the counter has its voltage regulator (7805) onboard, it may be supplied with anything (DC) from 8 - 30 V. Current consumption is 30 mA (no lcd backlight) up to 176 mA (R11 = 6.8 Ω, bright lcd backlight).

The workhorse of this circuit is unquestionably the Prescaler, who delivers 'manageable' frequency and amplitude towards the atmega.

DIY Mini VHF Frequency Display

Operating window of the prescaler. Drawing courtesy of ON Semiconductor.

This downscaled frequency is stable in amplitude, but needs a small buffer to achieve TTL levels. Our standard circuit with the BFR92A takes care of this job. If there is no signal present at the input, the prescaler may oscillate. To stop this, a resistor of 100 kΩ is soldered directly on C1. This will shift the bias point of the input stage slightly.

DIY Mini VHF Frequency Display

With a 12.288 MHz crystal, all devices were able to measure f ≥ 400 MHz accurately.

DIY Mini VHF Frequency Display

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t1 = 6585 d

t2 = 191 ms

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