13-190
TA0038
13
Copyright 1997-2000 RF Micro Devices, Inc.
data patterns. The slew rate due to the PLL loop band-
width is also evident here. the high loop bandwidth
offers a couple of advantages. First, the close-in phase
noise of the VCO is corrected for by the PLL, at best to
that of the crystal. More importantly, though, is that the
PLL synthesizer will have a faster lock time because of
the higher bandwidth. This is useful for systems that
occasionally burst a packet of data and then shut down
to conserve battery life. Burst type systems are also
affected by load pulling when the PA is turned on. The
faster loop response can minimize or eliminate out of
band spurs due to turn on transients that would be
problematic for low data rate systems that use lower
loop bandwidths.
For this design, the loop bandwidth is set at 50kHz and
a type II, 3rd order loop is used to help with suppres-
sion of the crystal reference spurs. Please refer to
technical article TA0031 in the 1999 RFMD catalog for
more information on designing the loop filters. Figure 6
demonstrates the close in phase noise and Figure 7
shows the spurious output. Also seen in Figure 7 are
the modulation spurs that result from the counter
cycles. The primary source for these modulation spurs
are voltage spikes caused by the gates and flip-flops
changing states during the carry over which then get
coupled into the VCO. Careful isolation and decoupling
are needing to the best suppression of these spurs.
The modulated signal, shown in Figure 8, shows an
occupied bandwidth (99%) of 150kHz for the 72 kbps
data rate. The loop filter acts as a smoothing filter and
helps shape the frequency spectrum even though the
input signal is digital. This bandwidth works well with a
receiver using low cost 180kHz ceramic IF filters.
The RF2513 transmitter IC has a companion single IC
receiver, the RF2917, for low cost FM or FSK simplex
links. The RF2917 incorporates the reference oscilla-
tor, PLL, VCO, LNA, downconverting mixer, 94dB limit-
ing IF amps, frequency discriminating demodulator,
and data slicer in a single chip. With the addition of a
few external, discrete components, a complete FSK
receiver can be built. One of the features of this part is
the differential connection of the demodulator to the
data slicer. A DC averaging circuit is not needed, there-
fore, the response to receiver data is 1 or 2 bit times
and a lengthy preamble is not required. This approach
is also immune to the low frequency components in the
data pattern that plague the DC averaging circuits. This
makes it a perfect receiver to complement the DC -
accurate modulator. A fully assembled evaluation
board for the RF2917 is available with 180kHz IF fil-
ters. By using this bandwidth for low data rates, sensi-
tivity is lost due to the increased noise power over the
signal power resulting in less than optimum range. The
higher bandwidth and larger deviations do not reduce
the frequency accuracy needed, thereby allowing
lower-cost crystals to be used.
There are a variety of applications that can benefit from
this type of modulation. Computer peripherals, for
example, tend to send data in bursts with indetermi-
nate pauses in-between. Wireless keyboards, mice
and joysticks are migrating from infra-red to RF, but the
immunity to random data patterns and pauses is nec-
essary to guarantee reliable operation. It is generally
preferred that remote controls for satellite receivers be
RF as opposed to infra-red so that the TV does not
have to be co-located with the receiver. The range of
infra-red remote controls can be augmented by RF
repeaters. An IR receiver data output can be used to
modulate this RF2513 circuit and an RF2917 receiver
data output can be buffered to drive the IR LEDs.
Telemetry sensor outputs such as temperature or fluid
level can be transmitted directly from the A/D output
with this transmitter without concern for the data pat-
tern. This modulator can also be implemented with the
RF2905 transceiver IC to create a low cost half duplex
two way radio link.
In conclusion, the enticements of low cost single chip
solutions get many people entangled by the problems
that random data patterns can cause when modulating
VCOs. Veteran radio designers are aware of the prob-
lem and several methods for working with it. With so
many new applications that need low to moderate data
rates at a low cost, this modulator provides a low cost,
easy to implement alternative to existing schemes.
This modulation scheme is being patented and will be
incorporated into new IC which can reduce the overall
cost of the transmitter even lower.
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
