VFO4351A 140MHz~4.4GHz RF signal generator based on ATMEGA 8A and ADF4351

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To replace my aging and bulky National Panasonic VP-8177A function generator, I bought the VFO4351A 140MHz~4.4GHz RF signal generator module from eBay. The design is based on an ADF4351 Wideband PLL Synthesizer, running on an ATMEGA 8A with a 25MHz crystal. Note the wrong frequency unit on the display (MHz not mHz):


The ATMEGA, ADF4351 and the crystal is at the back of the PCB:

PXL_20230324_073010234 (1)

it’s pretty straight forward to use the unit. Just apply 5V via the USB power port, enter the frequency you want to set and press OK. Use the ++ key to increase the frequency by a predefined step and — key to increase. To adjust the frequency step, use the <> key. To clear the input, use the C key. I couldn’t find out how to use the sweep features and I am not sure if this module supports such a feature.

Despite the indicated frequency range, my unit works well down to approximately 30MHz, below which there will be no RF output. The peak-to-peak voltage of the output is around 1.5V, which remains fairly constant in the low range and decreases as the output frequency increases. This is the 50MHz output as captured on my Rigol oscilloscope – 1.52V Vpp, 4ns rise time and 3.8ns fall time:


This is the 100MHz output with Vpp = 944mV, 3.3ns rise time and 3.5ns fall time:


The Rigol oscilloscope I am using is the DS1054Z, originally 50MHz, hacked to 100MHz. The rated bandwidth of an oscilloscope is the 3dB point, e.g. the point at which the input signal would be  attenuated to around 70% of the original. Hence, the measured Vpp, rise and fall time may not be accurate. This is the output signal as shown on a Hantek DSO4084C, hacked to 250MHz, showing a more accurate measurement of 1.19V for Vpp, 2.32ns for rise time and 2.36ns for fall time.


This is the 250MHz output as captured by the Rigol, with Vpp=460mV, 1.3ns rise time and 1.3ns fall time:


With such a high frequency, the Hantek also shows some attenuation – Vpp is measured as 624mV with 1.24ns rise time and 1.24ns fall time:


Remarkably, the hacked Rigol DS1054Z, rated for 100MHz can still correctly trigger on the 500MHz output from the RF module. Although the signal is heavily attenuated (Vpp=39.2mV), rise time and fall times are still (presumably) correctly measured (< 900ps for both). The Hantek DSO4084C can’t tell this input signal from noises.


At 666MHz, although the Rigol can still display what resembles a sine wave from the input signal, the frequency measurement is incorrect. You can see in the photo below, the counter displayed  333MHz (Vpp=42mV):


The limited bandwidth of the scope means that aliasing happened and not enough sampling points had been captured. The built-in frequency counter, working on these limited data points, arrived at a frequency of 333MHz, half the expected value. Notwithstanding the heavy attenuation, aliasing, and resulting wrong measurements, signals up to 1GHz from the RF generator can still be somewhat displayed on the DS1054Z. The DS1054Z, hacked to 100MHz, is a rock solid scope for hobbyists, in my opinion.

This is a more obvious example of aliasing due to lack of bandwidth on the Rigol oscilloscope:


Unfortunately, as I sold my 500MHz HP digital storage oscilloscope a few weeks prior, the only way for me to test this RF generator at higher frequencies is to use a frequency counter. The Victor VC3165 can count correctly a 2.3GHz signal from this RF generator:

PXL_20230324_072617197 (1)

A 1GHz signal works well too:

PXL_20230324_094850200 (1)

A 100MHz signal shows as 99.99645MHz on the VC3165 counter, which means that the accuracy should be at least 2 decimal places, quite good for a $30 module:


In general, I believe the VFO4351A RF generator module is quite worth the price and a must-have for a hobbyist’s home electronics lab.

See also:
Rigol DS1054Z, GW-INSTEK GDS1054B and Hantek DSO4084C digital oscilloscopes


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A tough developer who likes to work on just about anything, from software development to electronics, and share his knowledge with the rest of the world.

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