volume55-number1 - Flipbook - Page 11
The second long pulse FFT example in Figure 9 illustrates how the long PRI (low
PRF) results in very close spectral lines, which requires very low FFT bin size or
resolution bandwidth. The trade-off is even more time required (FFT N). A benefit
is even better sensitivity.
Where PRF (dBm) is the ADC input RF level at which the IMD3 and IMD2 levels
are measured.
Note that the cascaded system NF of the combination front end and ADC is
broadband noise prior to adjusting for processing gain.
Wideband Digital Receiver RF Front-End Design
Using a Cascaded ADC
Design Example of Front End to ADC Cascade
An example cascade analysis follows using the front end shown in Figure 10.
This chain benefits from the latest ADI releases to the RF catalog, including:
With dynamic range and sensitivity goals established, an RF front end must be
paired with the digital data converter. The optimal RF front end sets the receiver
sensitivity (NF) and performs the required spectral signal conditioning with good
enough linearity head room to allow the ADC performance to set receiver IP3
and IP2. Front-end RF gain is typically set to be good enough to establish the
required cascaded NF, as gain beyond that generally hurts dynamic range and
is avoided. It is criminal if the front end bottlenecks dynamic range and ADC
capability is thrown out!
Amplitude
1.5
ADMV8818 wideband programmable high-pass/low-pass tunable filter.
X
ADRF5730 wideband RF SOI digital attenuator.
X
ADRF5020 wideband RF SOI SPDT.
X
ADL8104 ultrahigh IP2 wideband RF amplifier.
X
AD9082 MxFE 4× DAC (12 GSPS) + 2× ADC (6 GSPS)
Additionally, the chain features a wideband 200 W RF limiter and small form
factor high Q fixed filtering developed at ADI.
A helpful trick is to convert the ADC figures of merit to equivalent RF cascade
parameters and treat the ADC like an RF black box. Some rules of thumb:
ADC NF dB = ADC NSD (dBm/Hz) + 174 (dBm/Hz)
ADC IIP2 dBm = 2PRF (dBm) – IMD2 (dBm)
ADC IIP3 dBm = [3PRF (dBm) – IMD3 (dBm)]/2
X
An age-old technique to preserve dynamic range is to switch between a high
sense mode for lower input signals and bypass mode for higher input signals.
As shown in Table 2, the high sense path favors NF performance, and the
bypass path concedes higher NF in favor of higher linearity (IP2 and IP3). The
performance tables illustrate this benefit.
(11)
PRI
1.0
PW
0.5
0
–0.5
NMtS
0
1k
2k
3k
4k
5k
6k
7k
Time (µs)
Magnitude (dB)
0
–50
–100
–150
–0.2
Noise
Floor
Main Lobe = 2/PW
–0.15
–0.1
–0.05
0
0.05
0.1
0.15
0.2
Frequency (MHz)
Magnitude (dB)
PRF
0
–20
–40
–60
–80
–100
–120
Parameter
Value
Units
FSAMPLE
15.36
GSPS
N
65,536
M
1536
M×N
101M
FFT Bin
0.153
kHz
Time (NMtS)
6.6
ms
PW
10
µs
Duty
1
%
PRF
1
kHz
PRI
1
ms
–126
dBFS
Noise Floor
Line Resolution
–4
–3
–2
–1
0
1
2
Frequency (MHz)
3
4
×10–3
Figure 9. A longer FFT of pulsed example to resolve spectral lines.
High-Pass Filter Low-Pass Filter
Digital
Attenuator
SPDT
SPDT
1
C
C
2
ADRF5730
ADRF5020
Balun
ADC
1
ADMV8818
Band-Pass
2
RF Limiter
ADL8104
ADL8104
ADRF5020
AD9082
Figure 10. Example RF front end featuring switched high sensitivity and bypass modes.
Analog Dialogue Volume 55, Number 1
11
