RF2516
With these known, it is then possible to determine the values of the filter components.4
1 + (ωLBW ⋅ τ2)2
τ1 KPD ⋅ KVCO
---- -----------------------------
C1 =
⋅
⋅
----------------------------------------
ω2LBW ⋅ N
1 + (ωLBW ⋅ τ1)2
τ2
τ2
τ2
R2 = -----
C2
⎛
⎞
C2 = C1 ⋅ ---- – 1
⎝
⎠
τ1
As an example, consider a loop bandwidth of 50kHz, a phase margin of 45°, a divide ratio of 64, a KVCO of 20MHz/V, and a
KPD of 100μA/2πrad. Time constant τ1 is 1.31848μs, time constant τ2 is 7.68468μs, C1 is 20.9pF, C2 is 100.8pF, and R2 is
76.2 kΩ.
In order to perform these calculations, one will need to know the value of two constants, KVCO and KPD. KPD is calculated by
dividing the charge pump current by 2π. For the RF2516, the charge pump current is 100μA. KVCO is best found empirically as
it will change with frequency and board parasitics. By briefly connecting pin 14 (LOOP FLT) to VCC and then to ground, the fre-
quency tuning range of the VCO can be seen. Dividing the difference between these two frequencies by the difference in the
voltage gives KVCO in MHz/V.
The control lines provide an interface for connecting the device to a microcontroller or other signal generating mechanism. The
designer can treat pin 8 (MOD IN), pin 16 (DIV CTRL), and pin 3 (PD) as control pins whose voltage level can be set. The lock-
detect voltage at pin 15 (LD FLT) is an output that can be monitored by the microcontroller.
Pin 15 (LD FLT) is used to set the threshold of the lock-detect circuit. A shunt capacitor is used to set an RC time constant with
an on-chip series 1kΩ resistor. The time constant should be approximately 10 times the reference period.
General RF bypassing techniques must be observed to get the best performance. Choose capacitors such that they are series
resonant near the frequency of operation.
Board layout is always an area in which great care must be taken. The board material and thickness are used in calculating the
RF line widths. The use of vias for connection to the ground plane allows one to connect to ground as close as possible to
ground pins. When laying out the traces around the VCO, it is desirable to keep the parasitics equal between the two legs. This
will allow equal valued inductors to be used.
Pre-compliance testing should be performed during the design process. This can be done with a GTEM cell or at a compliance
testing laboratory. It is recommended that pre-compliance testing be performed so that there are no surprises during final
compliance testing. This will help keep the product development and release on schedule.
Working with a laboratory offers the benefit of years of compliance testing experience and familiarity with the regulatory
issues. Also, the laboratory can often provide feedback that will help the designer make the product compliant.
On the other hand, having a GTEM cell or an open air test site locally offers the designer the ability to rapidly determine
whether or not design changes impact the product's compliance. Set-up of an open air test site and the associated calibration
is not trivial. An alternative is to use a GTEM test cell.
After the design has been completed and passes compliance testing, application will need to be made with the respective reg-
ulatory bodies for the geographic region in which the product will be operated to obtain final certifications.
7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical
support, contact RFMD at (+1) 336-678-5570 or sales-support@rfmd.com.
Rev A17 DS060712
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RFMD [ RF MICRO DEVICES ]
