## 4KHz-170MHz Wide Band RF Signal Generator – I

A signal generator can come in handy when working with digital and analog circuits. While most commercial grade signal generators offer more than just outputting a sinusoidal waveform, they are quite pricey for casual use. In this article, I will show you a simple wide band signal generator that is built around a Linear Technology’s LTC6905 Silicon Oscillator, which is can generate frequencies from roughly 17MHz to 170MHz. Along with a FairChild Semiconductor’s 74VHC4040 12-stage high speed binary counter, this signal generator is capable of generating precise frequencies from as low as 4KHz all the way through the ranges of LTC6905. The output waveform is either square wave (via the output from 74VHC4040) or sinusoidal wave (via LTC6905’s output), but should be sufficient for using as reference signals.

The following is the circuit diagram for this wide band signal generator:

4KHz to 170MHz Signal Generator Schematic

The frequency of the LTC6905 output is determined by the divider (pin 4) setting along with the voltage applied on pin 3. To generate more stable and accurate output, I used an Analog Devices’ AD5235 1024 position digital potentiometer for the frequency adjustment. In general, digital potentiometer of at least 8 bits resolution is desired (AD5235 has a 10bit resolution) since we can get closer output frequency steps. To further automate the frequency range selection of LTC6905 (the frequency ranges are selected via the settings of the divider pin), I used a reed relay switch along with a Texas Instrument’s TS5A23159 low on-resistance analog switch to control the transitions among the three divider settings LTC6905 has to obtain a full range of the available frequencies continuously. The frequency transition is controlled by a switch (SW1) along with an ATmega328p MCU.

In order to provide frequency ranges below LTC6905’s lower range (17MHz), I used a 74VHC4040 high speed 12-stage binary counter as a frequency divider so frequencies of $f_n = \frac{f_0}{2^n}$ can be obtained (n = 1 to 12).

One thing to keep in mind is that the output from LTC6905 is sinusoidal while the output from the frequency divider is square wave. This in general should not be of any issue when the output is used as a reference signal. In applications where the waveform requirement is strict, a high speed Schmitt Trigger may be added to LTC6905’s output.

Here are a couple of pictures showing the layout of the circuits:

Frequency Generator 1

In the picture above, the board on the top panel consists the MCU and the 74VHC4040 and the board inside the casing consist the rest of the circuitry.

The picture below shows the layout of the signal generator. Because of the high frequencies it is operating under, I chose to enclose the whole circuit inside an aluminum case to reduce possible RF interference and thus makes the circuit more stable.

Frequency Generator 2

In my next post, I will explain the code a little bit and show a few more pictures of this RF signal generator in action.

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1. Bill Moritz says:

Hello,

No site seems to stock them.
Can you help me?
Thank you.

2. So3ody says:

” One thing to keep in mind is that the output from LTC6905 is sinusoidal while the output from the frequency divider is square wave.”

i looked at the Data sheet and actually on page 1 its written “see Diagram 1 under Typical application P.1 ” that the output of Pin 5 is a Squarewave.

• kwong says:

You are correct, I checked the datasheet again and it did say that the output is squarewave. I guess that the sinusoidal output was an artifact of the scope (or possibly the capacitance of the output) I was using at such high frequency.

3. So3ody says:

did you try this circuit ? did it work ?

4. norbie says:

Howdy,
how stable was the frequency output.
Is it useful for example on an receiver or does it drift too much for this?
I believe it said +10dBm output, perhaps one could get crazy, shield it and put an attenuator at the output.
Think that the oscillator signal probably would be too strong.

In any case interesting project.
TNX

• kwong says:

The signal stability depends on the stability of the voltage divider that sets the operating frequency. When a digital pot is used (with a voltage reference), the stability is quite good.

5. uri Boros says:

Hi,

I am interesting for build wide band RF generator from 20Mhz to 140Mhz that genarete the signal to all the range without step like each 1MHZ
is anyone can help
i new in the market :)

Thank You