35MHz-4.4GHz digitally
controlled oscillator
Using Cheap Asian Electronic Modules Part 16: by Jim Rowe
Fig.1: block diagram of
a basic phase-locked
loop. They’re typically
used to generate a
stable high-frequency
signal from a fi xed low-
frequency signal.T
hat’s an impressive range of fre-
quencies that can be produced by
this surprisingly compact (48 x 36.5 x
10mm) module. It is available from var-
ious Chinese websites including Bang-
good (http://bit.ly/pe-may19-rf), Al-
iExpress and eBay, for around £12.50.
It’s essentially a smaller, lower-cost
version of the ADF4351 development
board sold by Analog Devices. It runs
from 5V and has two RF outputs, one
180° out of phase with the other, allow-
ing it to produce either single-ended or
differential signals.
It’s controlled using a serial bus
that’s connected via a 10-pin header,
which also makes connection to the
3.3V supply rail.
The ADF4351 chip at the heart of the
module is an advanced phase-locked
loop (PLL) device. Before we delve
into how the ADF4351 works, it’s a
good idea to briefl y cover the opera-
tion of PLLs.
Fig.1 shows the block diagram of a
basic PLL. It incorporates a negative
feedback loop, similar to the one used
to improve the performance of audio
amplifi ers. However, in this case, rath-
er than having a voltage divider pro-
viding the feedback signal, we have a
frequency divider in the loop.
The PLL’s output signal (FOUT) is
produced by the voltage-controlled
oscillator (VCO) at upper right. The
frequency divider divides this output
frequency by a factor of N.
The resulting signal (FFB) is then fed
to the negative input of phase detec-
tor PD, which compares its frequency
and phase with FREF, the signal from a
low-frequency reference oscillator, fed
to its positive input.
The PD output ‘error’ pulses are fed
to charge pump CP, which uses them to
develop a fl uctuating DC voltage with
a polarity and amplitude proportional
to the frequency/phase differences be-
tween FREF and FFB. This voltage is then
low-pass fi ltered and used to control
the VCO’s frequency.
This feedback action causes the
VCO frequency (FOUT) to stabilise at
very close to N times the reference fre-
quency, FREF. The PLL is then described
as being ‘in lock’, since the feedback
action keeps FFB locked to FREF in both
frequency and phase.
So even if FREF is fi xed, by changing
the division ratio N, we can control
the frequency of FOUT. Basic PLLs like
this have been in use for many decades
but more elaborate versions have also
been developed, to overcome some of
the limitations of a basic PLL.
One of these limitations is that the
minimum change in FOUT is equal to
FREF, so you need quite a low reference
frequency to have fi ne control over the
output frequency.
But it’s easier to produce accurate
and stable reference oscillators at
higher frequencies, so one of the fi rst
enhancements to PLLs was to add a
reference frequency divider between
the FREF input and the phase detec-
tor PD.Also, if the output frequency needs
to be up in the GHz (gigahertz) range,
it’s not easy to provide a programmable
divider working at these frequencies.
So another early PLL improvement
was to add a fi xed ‘prescaler’ to the
feedback loop, between the VCO out-
put and the input of the main (program-
mable) feedback divider.
Unfortunately, this reduces the out-
put frequency adjustment resolution.
However, this can be overcome by
adopting what’s referred to as a ‘dual
modulus prescaler’.
This is essentially a prescaler with
a division ratio that can be switched
from one value (say P) to another (like
P+1) by an external control signal.
We don’t have space here to fully
explain the operation of modern (and
quite elaborate) PLLs, but the prior de-
scription should be enough to under-
stand how the ADF4351 works.Inside the ADF4351
The block diagram of the ADF4351
IC (Fig.2) is somewhat more complex
than the basic PLL shown in Fig.1. The
VCO part of the device is labelled ‘VCO
CORE’ and shaded pink.
There are actually four VCOs inside
the core, each used to generate a differ-
ent frequency range. They are all tunedThis programmable frequency module is
based on the ADF4351 PLL (Phase-Locked
Loop) IC and it can produce a sinewave
from 35MHz to 4.4GHz, with crystal
accuracy. It can even be used as a sweep
generator and costs around £12.50.