diff --git a/makefile b/makefile
index 0f22b16..a8e5ce8 100644
--- a/makefile
+++ b/makefile
@@ -24,7 +24,7 @@ CFLAGS	= -Wall $(VER) $(incdefs) $(DEBUG) $(EXTRA_CFLAGS)
 LDLIBS	= -lm -lrt $(EXTRA_LDFLAGS)
 PRG	= ptp4l pmc phc2sys hwstamp_ctl phc_ctl timemaster
 OBJ     = bmc.o clock.o clockadj.o clockcheck.o config.o fault.o \
- filter.o fsm.o linreg.o mave.o mmedian.o msg.o ntpshm.o phc.o \
+ filter.o fsm.o linreg.o mave.o mmedian.o msg.o ntpshm.o nullf.o phc.o \
  pi.o port.o print.o ptp4l.o raw.o servo.o sk.o stats.o tlv.o \
  transport.o tsproc.o udp.o udp6.o uds.o util.o version.o
 
@@ -49,8 +49,8 @@ ptp4l: $(OBJ)
 pmc: msg.o pmc.o pmc_common.o print.o raw.o sk.o tlv.o transport.o udp.o \
  udp6.o uds.o util.o version.o
 
-phc2sys: clockadj.o clockcheck.o linreg.o msg.o ntpshm.o phc.o phc2sys.o pi.o \
- pmc_common.o print.o raw.o servo.o sk.o stats.o sysoff.o tlv.o \
+phc2sys: clockadj.o clockcheck.o linreg.o msg.o ntpshm.o nullf.o phc.o \
+ phc2sys.o pi.o pmc_common.o print.o raw.o servo.o sk.o stats.o sysoff.o tlv.o \
  transport.o udp.o udp6.o uds.o util.o version.o
 
 hwstamp_ctl: hwstamp_ctl.o version.o
diff --git a/nullf.c b/nullf.c
new file mode 100644
index 0000000..5512837
--- /dev/null
+++ b/nullf.c
@@ -0,0 +1,79 @@
+/**
+ * @file nullf.c
+ * @brief Implements a clock servo that always set the frequency offset to zero.
+ * @note Copyright (C) 2015 Richard Cochran <richardcochran@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+#include <stdlib.h>
+#include <math.h>
+
+#include "nullf.h"
+#include "print.h"
+#include "servo_private.h"
+
+struct nullf_servo {
+	struct servo servo;
+};
+
+static void nullf_destroy(struct servo *servo)
+{
+	struct nullf_servo *s = container_of(servo, struct nullf_servo, servo);
+	free(s);
+}
+
+static double nullf_sample(struct servo *servo, int64_t offset,
+			   uint64_t local_ts, double weight,
+			   enum servo_state *state)
+{
+	if (!offset) {
+		*state = SERVO_LOCKED;
+		return 0.0;
+	}
+
+	if ((servo->first_update && servo->first_step_threshold &&
+	     servo->first_step_threshold < fabs(offset)) ||
+	    (servo->step_threshold && servo->step_threshold < fabs(offset))) {
+		*state = SERVO_JUMP;
+	} else {
+		*state = SERVO_UNLOCKED;
+	}
+
+	return 0.0;
+}
+
+static void nullf_sync_interval(struct servo *servo, double interval)
+{
+}
+
+static void nullf_reset(struct servo *servo)
+{
+}
+
+struct servo *nullf_servo_create(void)
+{
+	struct nullf_servo *s;
+
+	s = calloc(1, sizeof(*s));
+	if (!s)
+		return NULL;
+
+	s->servo.destroy = nullf_destroy;
+	s->servo.sample = nullf_sample;
+	s->servo.sync_interval = nullf_sync_interval;
+	s->servo.reset = nullf_reset;
+
+	return &s->servo;
+}
diff --git a/nullf.h b/nullf.h
new file mode 100644
index 0000000..9e8bc7b
--- /dev/null
+++ b/nullf.h
@@ -0,0 +1,26 @@
+/**
+ * @file nullf.h
+ * @note Copyright (C) 2015 Richard Cochran <richardcochran@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+#ifndef HAVE_NULLF_H
+#define HAVE_NULLF_H
+
+#include "servo.h"
+
+struct servo *nullf_servo_create(void);
+
+#endif
diff --git a/servo.c b/servo.c
index 2739ebd..b936a85 100644
--- a/servo.c
+++ b/servo.c
@@ -20,6 +20,7 @@
 
 #include "linreg.h"
 #include "ntpshm.h"
+#include "nullf.h"
 #include "pi.h"
 #include "servo_private.h"
 
@@ -43,6 +44,9 @@ struct servo *servo_create(enum servo_type type, int fadj, int max_ppb, int sw_t
 	case CLOCK_SERVO_NTPSHM:
 		servo = ntpshm_servo_create();
 		break;
+	case CLOCK_SERVO_NULLF:
+		servo = nullf_servo_create();
+		break;
 	default:
 		return NULL;
 	}
diff --git a/servo.h b/servo.h
index 4be8c23..0a182a4 100644
--- a/servo.h
+++ b/servo.h
@@ -56,6 +56,7 @@ enum servo_type {
 	CLOCK_SERVO_PI,
 	CLOCK_SERVO_LINREG,
 	CLOCK_SERVO_NTPSHM,
+	CLOCK_SERVO_NULLF,
 };
 
 /**