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LT5504

800MHz to 2.7GHz RF Measuring Receiver

Linear Technology 

LT5504

APPLICATIO S I FOR ATIO

2.5

fRF = 1.9GHz

2.0

WITH SINGLE-ENDED-TO-

DIFFERENTIAL INPUT CIRCUIT

1.5

1: 63.56Ω

–j98.05Ω

900.00MHz

2: 26.69Ω

3

–j42.90Ω

1.90GHz

3: 28.88Ω

–j27.76Ω

1

2.50GHz

2

START: 100MHz

STOP:3GHz

5504 F01

Figure 1. Differential RF Input Impedance

RF

INPUT

MATCHING NETWORK

CS1

3.3pF

TO RF+

CS2

3.3pF

LSH

3.3nH

TO RF–

5504 F02

Figure 2. RF Input Matching Network at 1900MHz

Figure 3 shows the output voltage vs RF input power

response for these two input terminations. The voltage

gain of the single-ended-to-differential conversion circuit

is:

GAIN = 20 • LOG RIN = 3dB,

50

where RIN = 100Ω is the narrow band input impedance.

Thus, the output voltage curve in this case is shifted to the

left by about 3dB.

Table 1. The Component Values of Matching Network LSH, CS1

and CS2

fIF (MHz)

LSH(nH)

CS1/CS2(pF)

900

12.0

3.9

1900

3.3

3.3

2500

2.7

2.2

2700

2.4

1.5

6

1.0

WITH 1:1 INPUT TX

0.5

0

–80 –70 –60 –50 –40 –30 –20 –10 0 10

PIN (dBm)

5504 F03

Figure 3. The Output Voltage vs RF Input Power

Limiting Mixer and LPF

The amplified RF signal is down-converted using the

limiting mixer and LO signal. The resulting signal is filtered

by the 3rd-order, 450MHz, integrated low pass filter (LPF).

Only the desired IF signal is passed to the IF limiters for

further detection. Any other mixing products, including LO

feedthrough, are much reduced to maximize sensitivity.

The receiver’s sensitivity is thus defined by the LPF band-

width.

IF Limiter

The IF signal is then amplified through the multiple limiter

stages for further signal detection. All DC offsets, includ-

ing LO signal self-mixing, are eliminated by an internal DC

offset cancellation circuit. Nevertheless, care should be

taken in component placement and in PCB layout to

minimize LO coupling to the RF port.

Output Interface

The output interface of the LT55O4 is shown in Figure 4.

The output currents from the RF and IF detectors are

summed and converted into an output voltage, VOUT.

The maximum charging current available to the output

load is about 400µA. An internal compensation capacitor

CC is used to guarantee stable operation for a large

capacitive output load. The slew rate is 80V/µs and the

small signal output bandwidth is approximately 5MHz

when the output is resistively terminated. When the output

is loaded with a large capacitor CL, the slew rate is limited

5504f

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